Karl,
I just tried a couple more, and found that my 1908 DWM comes up slightly short on a Mauser frame.
You are correct that filing the rear edge to the rounded contour will solve the problem.
It appears that the curve at the rear inside of the frame must have some variance from early to late and mfg. to mfg!
Thanks for you pictures and input. It is a good day when one learns something new or different, especially with all the ins and outs of lugers.
Miles,
check for full travel, and if need be round the rear edges of the rails on the upper!