I've got a crazy desire to buy a 4" 629, but I have no idea which -# to look for. Can anyone outline what the major changes were in the different generations? I'm particularly interested in when the firing pin moved from the hammer to the frame, when they introduced MIM parts and of course, when they introduced the IL.

Can`t help you on the major changes but if can find a 4in.629 P&R like this one ,BUY IT.
Dick

They were introduced in 1979 at serial number N748564
1981 introduce 4" and 8 3/8" barrels
629-1 Eliminate counter bored cylinders and pinned barrel
629-1 (1986-87) 8,000 made with round butt and 3" barrel, Distributed by Lew Horton and S&W
629-2 New Yoke retention system, radius stud package, floating hand; intergral scope mounts available on 8 3/8 barrel
629-3 Endurance package; transitional changes; harden yoke/frame
629-4 (1993) Change rear site leaf, drill and tap frame, change extractor, Hogue grips introduced
629-4 (1996) Round Butt production only; ship in blue plastic box
629-4 (1997) change thumbpiece, Master trigger locks shipped
629-5 (1997) Change Frame design to eliminate cylinder stop stud; eliminate serrated tangs; Change to MIM hammer and floating firing pin; change to MIM trigger; change internal lockwork
629-6 (2001) Introduce internal lock
629-7 (2005) new two piece barrel
629-8 (2006) slabside barrel

close as I could come

Thank you fyimo! That was exactly what I needed.