Having studied the drawing for a while, my best guess is that it must go together something like this:
There must be removable stub axles and hubs which are not shown which bolt onto the ends of the chassis, which makes sense if its designed to be broke down into manageable parts. Then as Martin says, the trails attach to the holes next in from the ends and the main gun body goes in the centre pivit which allows for the gun to be traversed. I think the forward protusion is to mount the main shield perhaps? and the two small brackets labeled i27 are to mount the lower shield... maybe!!? 
Γ’β¬ΕI wanted to come to the Volga at a specific location at a specific city. By chance it carries the name of Stalin himself. So donΓ’β¬β’t think I marched there for this reason Γ’β¬β it could carry another name Γ’β¬β but because there is a very important goal... this goal I wanted to take Γ’β¬β and you know Γ’β¬β we are very modest, we have it already."
Adolf Hitler, November 1942
"Comrades, Red Army men, commanders and political workers, men and women guerrillas! It is on your perseverance, staunchness, fighting skill and readiness to discharge your duty to the country that the defeat of the German-fascist army and the liberation of the Soviet land from the Hitlerite invaders depend! We can and must clear the Soviet land of Hitlerite vermin."
Joseph Stalin, November 1942
Actually it has just occurred to me that the bottom of the picture is the front rather than the back of the chassis. If you look closely at the photo of the gun in the field the wheel hubs do look to be out in front of the main chassis beam. The parts marked i27 may then be clips of some sort to lock the trails in the spread position!? Of course I could be talking complete bol**cks! 
Γ’β¬ΕI wanted to come to the Volga at a specific location at a specific city. By chance it carries the name of Stalin himself. So donΓ’β¬β’t think I marched there for this reason Γ’β¬β it could carry another name Γ’β¬β but because there is a very important goal... this goal I wanted to take Γ’β¬β and you know Γ’β¬β we are very modest, we have it already."
Adolf Hitler, November 1942
"Comrades, Red Army men, commanders and political workers, men and women guerrillas! It is on your perseverance, staunchness, fighting skill and readiness to discharge your duty to the country that the defeat of the German-fascist army and the liberation of the Soviet land from the Hitlerite invaders depend! We can and must clear the Soviet land of Hitlerite vermin."
Joseph Stalin, November 1942
Some good idea's 
Not sure about the gun sheild though, remember that unlike a the normal version its a 2 piece left right sheild, in the picture below you can see the rods at the back which must hook onto the chasis.
Also in the pic below the recoil block seems to be about a foot of the Unterlaffette.
If Lutz or Gordon are about a translation of the drawing key's may help. If not I will attempt google translate may throw some light on things.
Ok, I think I have figured it.
Looking at the wheels, the hub is actually right there labeled as r20.
The bit labeled r12 goes into the holes in the end of the chassis.
The ends of brackets of the shield labeled as 062 slot into i27 on the chassis!
Γ’β¬ΕI wanted to come to the Volga at a specific location at a specific city. By chance it carries the name of Stalin himself. So donΓ’β¬β’t think I marched there for this reason Γ’β¬β it could carry another name Γ’β¬β but because there is a very important goal... this goal I wanted to take Γ’β¬β and you know Γ’β¬β we are very modest, we have it already."
Adolf Hitler, November 1942
"Comrades, Red Army men, commanders and political workers, men and women guerrillas! It is on your perseverance, staunchness, fighting skill and readiness to discharge your duty to the country that the defeat of the German-fascist army and the liberation of the Soviet land from the Hitlerite invaders depend! We can and must clear the Soviet land of Hitlerite vermin."
Joseph Stalin, November 1942
As far as the breech and the problem of loading a pyro in an elevated chamber are concerned what about cutting the rear lower section of the gun barrel off just slightly larger than a charge, this could then be welded to a steel breech block at the rear (non moving) section of the breech block. The charge could be placed in this section and the breech closed upon it. extra metal could be welded to the side off the cut off section of the barrel to help seal the blast. Of course the tendancy would be for the blast to open the breech but if the firing leaver was utilised for operating 2 sliding plates this would lock the breech together when firing?
Ok, I think I have figured it.
Looking at the wheels, the hub is actually right there labeled as r20.
The bit labeled r12 goes into the holes in the end of the chassis.
The ends of brackets of the shield labeled as 062 slot into i27 on the chassis!
Ah yes well done Bosh Of course unfortunately as I will be using small wagon wheels I will probably have to use a more conventional axel, but good to figure that out.
Also in the pic below the recoil block seems to be about a foot of the Unterlaffette.
If Lutz or Gordon are about a translation of the drawing key's may help. If not I will attempt google translate may throw some light on things.
Tim sling it across on a PM
As far as the breech and the problem of loading a pyro in an elevated chamber are concerned what about cutting the rear lower section of the gun barrel off just slightly larger than a charge, this could then be welded to a steel breech block at the rear (non moving) section of the breech block. The charge could be placed in this section and the breech closed upon it. extra metal could be welded to the side off the cut off section of the barrel to help seal the blast. Of course the tendancy would be for the blast to open the breech but if the firing leaver was utilised for operating 2 sliding plates this would lock the breech together when firing?
Thinking about this more, the shell idea might actually be the best way to go. I have found from using my 5cm mortar that you don't need a long barrel to get good range. So if you had a shell that was effectively a short mortar then you could model the shotgun breach as per the original but it wouldn't need to take any blast pressure. If you made the shells like I made my mortar tube, a thick walled 40mm ID steel pipe with a 5cm long solid billet bar in the end which is then cross-drilled and pined with a 10mm steel bar, they would be very solid and heavy, much heavier than the shells Josh used to use so no possibility of it going anywhere. You could safely hold it in your hand and fire off a round without any risk if you had to. So just pop the pryo in the shell, light the end, load it into the gun and close the breech. After firing, open the breech and the empty and smoking shell pops out the back, all very realistic! 
Γ’β¬ΕI wanted to come to the Volga at a specific location at a specific city. By chance it carries the name of Stalin himself. So donΓ’β¬β’t think I marched there for this reason Γ’β¬β it could carry another name Γ’β¬β but because there is a very important goal... this goal I wanted to take Γ’β¬β and you know Γ’β¬β we are very modest, we have it already."
Adolf Hitler, November 1942
"Comrades, Red Army men, commanders and political workers, men and women guerrillas! It is on your perseverance, staunchness, fighting skill and readiness to discharge your duty to the country that the defeat of the German-fascist army and the liberation of the Soviet land from the Hitlerite invaders depend! We can and must clear the Soviet land of Hitlerite vermin."
Joseph Stalin, November 1942
Hi Tim, hi Gordon,
Happy to help on translation. Let me know where i can help.
Cheers
Lutz
Thinking about this more, the shell idea might actually be the best way to go. I have found from using my 5cm mortar that you don't need a long barrel to get good range. So if you had a shell that was effectively a short mortar then you could model the shotgun breach as per the original but it wouldn't need to take any blast pressure. If you made the shells like I made my mortar tube, a thick walled 40mm ID steel pipe with a 5cm long solid billet bar in the end which is then cross-drilled and pined with a 10mm steel bar, they would be very solid and heavy, much heavier than the shells Josh used to use so no possibility of it going anywhere. You could safely hold it in your hand and fire off a round without any risk if you had to. So just pop the pryo in the shell, light the end, load it into the gun and close the breech. After firing, open the breech and the empty and smoking shell pops out the back, all very realistic!
Well it would certainly make construction of the breech easier and also I think would feel more realistic for the crew . Also had the idea that i would like it to pop off crow scarers for normal reenactment and the shell would certainly facilitate that.
