Hi,
That is quite a long story, but here is the 'quick version'.
The Mauser Parabellum was based on the Swiss 06/29 design and initially followed the Swiss 06/29 frame style (without the little bump on the front grip strap) and the Swiss frame ramp. Mauser Parabellums made in 1970, 71 and 72 have the Swiss frame style.
Interarms wanted a frame style that was closer to the original pre-1945 style and Mauser redid the frame, introducing it in late 1972.
The changes done to the grip style are purely cosmetic. Other changes to the frame were needed to enable production of a common frame for both 9x19 para and .30 luger versions with different barrel lengths.
An advantage of the Swiss style frames is that you can attach Swiss 06/29 grips to them to improve the 'feel' of the grips. The drawback is that the Swiss style frame doesn't give you much tinkering space for customizing barrels, calibres and barrel lengths. That's what Mauser found out
