If you haven't already and your car was originaly delivered to a Canadian Dealer, get ahold of GM Canada Vintage services to get a vehicle package for your Vin!.
When recorded and available, included in their info is RPO quantities by model of cars produced for sale in Canada...sometimes by engine RPO and/or body style, sometimes not..
GM Canada Vintage Services...ask for George!.
http://www.gm.ca/gm/english/shopping/parts/vintage
Not exact but a good rule of thumb sometimes too is multiplying known Canadian RPO production by 10 to arrive at a good guestimate of US production by RPO..
Welcome aboard!.
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~ Pete