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Wrong. The designations changed in '87.
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I am not saying I disagree with you, but If that is wrong, then why do all aftermarket people always break down their GM truck parts as 1973 - 1987, 1988 - 1991, and 1988 - 1998? Look at all the major aftermarket GM truck vendors like LMC or JC Whitney. They break thier parts down by those years. If the change was in 87, then shouldn't the break down be 1973 - 1986, 1987 - 1991, and 1987 - 1998?
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Because they're going by appearance. Usually I see 73-80 80-87 and 88-91. Mechanically all similar, visually different, and that's what most people see.