Using 3 images from the ESB stills I got a pretty good comparison between different takes, and it seems that all the major parts are/were in the same place, more exactly there were no major changes in position or order:
[ATTACH]40466[/ATTACH]
And here is a quick comparison of Alan's drawing with one of the images:
[ATTACH]40467[/ATTACH]
I agree with intwenothor that the MPP tube has 120mm sticking out from the Webleys' barrel.