I do not have a copy of the GCR, but in looking at the 1972 SCCA runoff results on the VIR History website, GP is all sprites, midgets, and spitfires, whereas BS is primarily Datsun 510s and BMW 2002s.
I believe back then SCCA production classes were limited to 2-seaters.
Here is the VIR page:
http://www.virhistory.com/cars/arrc/index.htm
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