I don't think GM stamped any numbers on the air cleaner housings or covers. As I remember they put stickers on the covers with the engine type and size and on the side of the lower housing with the part numbers. So many GM's had dual snorkle in the late 60's and early 70's that there should be a lot of them around. I'm amaized that more are not on Ebay.
Buick, Cadi' and Olds put them on their big motors on a lot of applications, Chevy put a lot of them on too.