I've read shallower too. They are longer for certain. The older models had notches that were symmetric with the relief cut, and the newer models have the cut extended on the forward end. It was explained that the bolt contact area was kept constant by making the bolt and notch longer and proportionally less deep.

The "Standard Catalog of Smith & Wesson, 3rd Ed." makes reference to a new yoke retention system, radius stud package, floating hand (1988), fixed hand (1989), hardening of the frame/yoke, bolt block added, longer stop notch, and internal lockwork changes including dimensions of parts and relocation of parts. Supica and Nahas give no details that I can find. Also, some changes were the result of changing to CNC manufacturing, and they don't differentiate.