Early Model (87-89) rear brakes - PITA

Early Model rear drum brake shoes and wheel cylinders appear to be different than any of the other model years. What a pain. I was doing a rear brake replacement and went to get new shoes and they told me they had to order them, I said really, they should be the same a a million other GM cars and vans.

Well when they came in we compared them to a set for a on the shelf and they are a bit diferent. See the pic

:link:

Now on to the wheel cylinders. Another PITA, they do not follow the same year break as the shoes do. Basically it says that a 1985 and 1986 use this and nothing else in the entire GM fleet. Also notice the long tab on the upper picture, well seems that is needed on the wheel cylinders they have for the 87-89 but not before or after those years., but the shoes I have don't have them and the current wheel cylinders I have would not accept the tabs either, and sicne 1985-86 wheels cylinder need to be ordered, whe looked at 1991 and I decided they where close enough.

So why did GM change the braks on the 87-89 and them went back, also any reason the 1991 should not work? I looked at master cylinders for those years and found that they are the same for 85-94, s I figure any wheel cylinder shoes set that matches up should be fine.