Alan, I don't agree. I looked at this in some detail a while back as part of the barrel mount found part discussion and I made it that the MPP tube should extend about 120 mm from the end of the Webley barrel (which is about 4 3/4"). That 120 mm includes the section which is hidden (or obscured) by the front barrel mount. Unfortunately I'm not capable of your standard of excellent drawing but I did have access to the gallery and my own real found parts (Webley/MPP etc) to come to this conclusion.
When I saw this thread I thought of something I'd seen before. Low and behold:
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:
And here is a quick comparison of Alan's drawing with one of the images:
I agree with intwenothor that the MPP tube has 120mm sticking out from the Webleys' barrel.
Thanks, Alan! I would love to have your technical drawing skills, because your drawings looks always amazing!
Whenever is possible, I try to use lots of images to confront my templates/stencils to achieve the best results. But sometimes, in lack of good reference material, you have to rely on your intuition... and hope to get it right.