[DonVoigt]

The long and short frame has nothing to do with interchange, a long receiver on a short frame will extend to the front- and function; likewise a short receiver will function on a long frame (not a 1900, but a 1906) and function, but will not line up with the front of the frame.

I have not experienced the problem that Karl mentions; the "reinforcement" to the luger frame is on the inside of the frame, and does not impact the receiver.

Just try it, a 1910 upper will fit on a 1918 lower without any problem, or a Mauser frame for that matter.