When BMW contracted with ZF for transaxles to be used in the M1, the agreement was, ZF would update and strengthen the units. More specifically, the case and input shaft were strengthened. Also, the gear and synchro ring design was updated. Oddly enough, they still didn't see fit to safety wire the ring gear bolts! This became the standard ZF transaxle from that point on. Years later it became the RBT 5 speed transaxle except the ring gear bolts finally came safety wired!
As far as gear ratios are concerned, the R&P was still a 4.22:1. The first three gear ratios were slightly lower (since the M1 had a high revving six cylinder engine) but 4th and 5th were both taller than in a "Pantera" -2. I think DeTomaso must have had a bunch of ZF -2's in inventory because the only Pantera's I've seen with the M1 version of the ZF were Si's.
When I had an M1 ZF prepared for use in my Pantera, I had Lloyd install a 3.77:1 R&P, since the last thing I wanted was lower gearing in the first three gears!