Here's a list of dash numbers. I forget the year the Chief's Special started (1950?), but the Model 36 officially started in 1957 and some of these are made at the same time, including Lady Smith versions.
1967 36-1 3" heavy barrel
1967 Change to forked mainspring strut
1968 Delete diamond grip
1969 Begin "J" serial prefix
1975 Heavy barrel standard
1982 Eliminate pinned barrel
1988 36-2 New yoke retention system on tapered bbl
1988 36-3 New yoke system heavy bbl
1989 36-4 2" Ladysmith tapered bbl
1989 36-5 3" Ladysmith heavy bbl
1992 Disc. 3" on Ladysmith
1994 Laminate rosewood grips, extractor change
1989 36-6 Target version (special production), 3" full lug, ribbed bbl, wide smooth trigger
1990 36-7 New sight width on standard frame and bbl
1990 36-8 New sight width on heavy bbl
1991 Ltd edition with full lug and target sights
1992 2" and 3" heavy bbl in round butt only
1994 Uncle Mike's grips, change extractor
1995-6 Delete square butt, only 2" in production
1996 36-9 Magnum frame intro for .38+P, MIM hammer and trigger, change lockworks
2001 36-10 Internal key lock for Ladysmith
2001 add fired case to box
2007 Model 36 Classic starts
Not sure when the flat latch disappeared, maybe 1965ish?