So I've finally managed to get a later Model M ('91) to compare it to my '86 one.
I really didn't expect to see many differences, but there were (as I do not have a camera at the moment, I can't take pictures, but I'll describe it all).
#1 chassis
--> the upper chassis is actually molded differently. The little dipped line (where dust and other stuff loves to get stuck) is actually much thinner, thus harder to clean on the later model. Also, the chassis imprint a lot more grainy... makes it feel more coarse, rather than buttery smooth on the '86.
#2 plastics
--> the chassis shell seems to be made of a different plastic. Feels way different (but this could be largely attributed to the grainier texture).
#3 key feel
--> the '91 model M definitely feels more stiff, and also more damp. The '86 is fairly loose, and the backboard reverberates way more. However, the looseness of the keys is NOT associated to use, because the '91 has been used a lot (the keys have shiny spots), but the '86 doesn't have a single key with a shiny spot at all, it simply wasn't used that much when I got it. This could be due to the '86 missing more rivets, but the differences may lie in the chassis itself.
#4 key labels
--> obviously this is already known, the '86 has a much bolder printing (but the green on the alt keys is actually a different colour, it's more yellow, I prefer the green on the '91 which has a tiny hint of blue opposed to yellow).
#5 LEDs
--> again, it's known they have different LEDs (I think the '91 LEDs look more green and less bright; haven't been able to turn both of them on at the same time due to lack of another SDL cable).
There are also colour differences, the plastic backing on the '86 is black, and on the '91 it's white (but that's really not significant to me), and the key stems vary colours on the '91.
In conclusion, the biggest difference (for me) was the different pattern molded into the chassis. I really prefer the smoother case of the '86. While key feel is slightly different, I think all Model Ms may have a variance in key feel... just because of so many factors involved.
