
There have always been strange conventions, and odd cars, etc (for example, my 1980 343 DL has everything a GL should have...in fact, by having a sunroof it's actually higher-spec than my 1981 343 GL), and there were subtle (or sometimes less subtle) changes with each model year. However, you can pretty much group them together into three basic bodystyles - when the most radical changes happened - mk1 (1976 - 1981), mk2 (1982 - 1985) and mk3 (1986 - 1991). The cars in each of these groups look the "most" like each other, basically, even if they're not exactly alike - the bumper change between 1976-1980 and 1981 model years being possibly the best example.
cheers
James