F3 is pretty easy to determine. Four (or five) digit code inside the film back where the hinge is located. First digit is month. Second digit is last digit of the year. Third digit is version of F3, fourth is revision. For regular F3 bodies, serial numbers up through 189xxxx are from between 1980 and 1989. 19xxxxx is 1990s, with the 20xxxxx serial range being 2000-04ish.
The F4 has a formula that is listed on the old Nikon F4 FAQ site, which has been archived, since the site owner shut it down. The formula isn't as accurate as the F3 method.
F6 bodies, you'd probably have to ask Nikon about.
The FM2/FM2n bodies use the same code system as what Nikon used on the F3, however, between the years 1986 and 1991 or so, the code sometimes had the first two digits reversed.
FM2 bodies in the 700-709xxxx range all seem to be from 1982. 710-715xxxx also from 1982. 72xxxxx starts in 1983 and continues through about mid 1984. Bodies above 724xxxx typically were the first FM2n bodies. 716-719xxxx are 1983. The 73xxxxx serial range started in August 1983 and continued through about 1987-summer 1988. 74xxxxx started in January 1985 and continued through August 1987. 75xxxxx started in September 1988 and continued through July 1991. 76xxxxx, June 1987-April 1989. 77xxxxx, July 1991-August 1995 or so. 78xxxxx, April 1989-around August 1990 or so. No 79xxxxx bodies were made. Lower serial 8xxxxxx bodies were all chrome. After the 85xxxxx serial block was introduced, Nikon's convention of alternating the second digit, depending on body color, disappeared. Sometime around the 785xxxx serial and 7538xxx serial block, Nikon switched to the N8008 style aluminum shutter curtains from the honeycomb pattern titanium curtains. More of a cost cutting measure than making the shutter stronger. The aluminum curtains do get as easily damaged as the titanium curtains.
-J