I can’t comment about the two-tone portion of cars (not paid enough attention to that aspect.
All ‘88-89 M-body’s were made at a different assembly plant than the ’87 and older ones were made at – so even though they are “mostly” the same, there are a lot of differences between some parts made between different vehicle assembly plants.
All ’88-89’s was made at the AMC plant in Kenosha, WI (’86?-89).
F and M-body’s before then, were made at St Louis, MO from ’76-87, at Newark, NJ (’77-80?), Hamtramck, MI (’76-80) and Windsor, Ont. (’81-83).
Some things like the rear doors (where the stationary glass and/or cover (for Fifth Ave)) and a few other areas of car, are almost 100% different.
I could be mistaken, but I think all cars with electric exterior mirrors came from Kenosha.
Also heard that the cars from Kenosha used better (rust resistant) steel in the body.
BudW