The shape and placement of the frame ears, where the toggle knobs strike the frame during recoil.
The Swiss decided to finetune it to their ammuntion on the 06/29 model. Mauser initially copied it and found out it gave problems with certain barrel configurations. In the end they fixed it by moving the frame ear surfaces 1mm to the rear.
If you compare a 29/70 frame and a 06/73 frame, the change will become obvious.
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