Not really a movie many of us in the UK know so had to look it up for a clip.
Maybe this may help answer.
http://www.youtube.com/watch?v=MF-d21YVh-Y
Free speech is expensive these days!
I'd have thought ammo supply would be an issue.
The .45ACP is a sub-sonic round, so a suppressed M3 would be able to used standard issue .45ACP ammo. 9mm Parabellum is supersonic, so unless the whole gun or barrel is made for suppression (like the "SD" variants of the MP5 are), you'd need special light-load ammo. That's the reason 9mm suppressors that work well are so long.
From what I can gather AB boots were polished black sometimes anyway.
Black dyeing of AB boots found here:
http://www.diggerhistory.info/pages-uni ... t-wear.htm
GS cap is shown further down the page you linked to. It's the nasty greeny grey cotton beret thing.
