This is actually the original seat that came with the truck when it was bought in late '84. My father-in-law picked it up at our local Chevy dealer. Traded an '82 longbed for it. Only truck I have ever seen this seat in.
I think that fabric style is "Deluxe Cloth" and it is done in "Dark Charcoal" - RPO code is 18D.
Your seat is an A52 style (just a plain old bench seat):
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Color chart for seat trim (1987):
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Parts list for A52 seat trim (I used the cushion list but the seat back P/N's are the same):
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Note that the Trim Material Type index at the top of the first page is screwed up. They only list three codes, when there should be four. There should be two types of cloth - Custom (C) and Deluxe (D). You can look down the and see the two vinyls (V & W) and also the two cloth fabrics.
I had to go to the 1984 parts list to figure it out:
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And even that was confusing because in 1985 they changed the names from Cloth (C) and Custom Cloth (D) to Custom and Deluxe.