I know others have experienced this problem, but is there a solution? The stock extension butt piece has jammed on one of my MP40s - i.e. it's locked in a "horizontal" position so it won't fit into the shoulder.
You've got nothing to ein, zwei, drei, vier
If you mean the short oval bit ...yes....I hit it and it behaved . If you mean the long bit then er no .....
Short oval bit - I didn't know what to call it, either! OK, brute force it is. 
You've got nothing to ein, zwei, drei, vier
Bit of WD40 and some firm handling sorted my seized butt piece......
You've got nothing to ein, zwei, drei, vier
Bit of WD40 and some firm handling sorted my seized butt piece......
Thought I'd logged on to an entirely differnet forum altogether then for a moment