Tech information furnished by Jim Chenoweth
Here is the real deal on 49 state US distributors from the beginning of
All early - 1987 have mechanical advance. Curves vary from year to year.
Early - 1968: Xtra small distributor, vacuum advance (correctly called "non-smog",
also sold as "non-USA").
>li>1969 - 1974: XS distributor, but w/ vac retard.
1975 & early 1976: small distributor, completely redesigned, very good quality
points distributor. Vacuum retard.
Late 1976 & 1977: Same, but w/ vac advance
1978: redesigned w/ Med. size screwdown, waterproof cap, electronic ignition. Vacuum advance & retard.
1979 - 1980: dual diaphragm advance. One big advance stage for normal operation,
small second stage for extra advance at hi-altitude
1981 - 1987 distributor body redesigned to use large cap. Same dual diapragm
advance introduced in '79.
1988 - 1992 distributor redesigned w/ out advance. Distributor pickup is a crank
angle sensor, advance curve is controlled by computer.