I just tried out of curiosity, and it fit. I did not try to shoot it though. I could see wanting a MKIII lower on a MKII upper, but not sure I would want to go the other way.
A Mark II upper on a Mark III lower, along with the Mark II's hammer and hammer bushing, would be the ideal Ruger Mark pistol. Nice mag release, no mag safety, and no trace of an LCI. Maybe call it the Mark II 1/2?
