So, I've actually got two of these blasters. They're currently in a state of flux, because I recently got some new parts. But yeah, one of the scopes I have is Vince's, and I get .759" for the diameter at the rear end. My real "end cap" is .754" at the base, and it has a very very slight taper (more of a curve, really), so if I measure up in the middle area of the cylindrical area, I'll get .745".
Yours looks right to my eyes, I think Vince made his "end cap" just a little on the small side, diameter-wise.
In this pic, the CE scope is on the bottom blaster, and it's got an inaccurate end cap. The top blaster has the new scope with correct end cap.
Here's another shot comparing the two. Note, I cut Vince's end cap off shortly after I received it, to install the correct one. But since then, I removed it again to install it on this new scope. So, Vince's original cap isn't in these pics, but the one I modeled as a replacement is very similar to his. I haven't gone back to change my model since getting the real end cap. Too many other projects, but I will remodel it to get as close as I can to the real one.
Oh, one other thing, to answer your question about the stock attachment nut. Mine are 5/8", or .625", or 15.85mm. I think that's pretty close to the original, but I'm not 100% sure, and I'm also not 100% sure about the internal threads. I think they're supposed to be 3/8-16. Can anyone confirm that?