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I think the 1969 Chevrolet Chassis Service Manual, Section 5, Brakes, has a table (figure) Called "Brake Main Cylinder Identification" In it, it lists the main cylinder Letter code and model applications and other info. I do not have a copy of the 1969 issue. Others may.
Just a thought.
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All I show is AT, AT,BS,DC,PG,TA,BA, and US.