This one is serial #0591, built 8/76, so it should be installed on a range of '76 model year Blazers, like several others I have in my mega-spreadsheet list of these. My best guess is that the Chinook company never tried doing this, however, the history of these is rather sketchy. To the best of my knowledge, Chinook wasn't doing anything with GM after early '77, but I'll welcome any info anybody has to clear more of the mysteries up. A rather good Chinook history is seen at Tin Can Tourists at
their page here.
When I talked to the seller, he told me its numbers, and that he had gotten it out of California. Decent possibility that some customizer modded it there, but it sure would be neat to extract more info out of this one's situation.