Hugh,
I am glad you could verify that a long frame and receiver can be cut back to short frame dimensions. The extra length of the long frame is all forward of the takedown lever, so all that is required is squaring up the forward part of the frame to remove the extra 2mm. This particular example was especially well done by carrying the radius smoothly into the squared off reduction. Properly done, I suspect that "clocking" the new barrel would be a relatively simple matter.
I believe that Dwight is right on the money that this was originally a 1906 First Issue Navy. I also think that the appearance that the reciever is a little bit shorter than the frame is due to the fact that the toggle action was not fully back into battery in the first photo. If you look at forward toggle link, it is canted and not parallel to the receiver rail (probably the recoil lever was not hooked to the mainspring bellcrank as well since the takedown lever isn't up and locked). Also if you look at the second photo, the frame and receiver line up just right. Your observation about the truncated front proof mark is just more verification of this rather well executed rework.
I am a little puzzled however by the gap between the rear toggle link and the receiver rails. Obviously the original Navy rear toggle link was replaced, but there shouldn't be any difference in overall length of the link.
