Buff there were a run of about 500 pistols that were essentially first run 4506's that were roll marked 645 on the frame. These guns were basically identical to the 4506 with the exception of the unique rollmark on the frame. I've never seen a 645 slide mounted on a 4506 frame to date, although I guess it's possible that such an animal exists considering all of the various version that Smith & wesson produced throughout those years.

I've seen at least 3 of these on gunbroker and the owners really think they have something special because all of them were asking in excess of $800 as a reserve. Now I'm not saying these aren't special but none of those guns sold so demand for them isn't that high it seems.