[Ron Wood]

Marc,
Worn or broken parts consideration aside, it is not an unsafe combination. The difference in frame and receiver lengths are all forward of the take-down bolt, so everything from there back is the same for all Lugers (possibly with very minor manufacturing differences from various makers). There is a slight difference between the M1900 trigger, sideplate and transfer lever and later models so that can cause function problems, and since there was some degree of hand fitting in Luger assembly it someimes is difficult to make a "parts gun" function. But none of that is due to the difference between a "long" or "short" frame/receiver.