Appreciate the responses. I think the as-cast versus machined/finished/assembled part number differences are at play here.
My drivers side manifold is cast -038. My pass side is cast -037. I'll try to post pics of them tomorrow.
In OE, the drivers side, once machined, is done and that's all you get.
In OE, the pass side, once machined, is then outfitted with the heat riser components and that may be what drives the machined part from -037 to -036.
This is somewhat supported when looking at the pricing on the attachment from Lamnas. Both of the RH manifolds for the round-port and D-port applications carry a price premium over the LH manifolds (which have no extra components attached to them).
In the aftermarket, the folks put the -038 and -037 part numbers in the castings (like I have), but the RH manifold is not machined for the heat riser components (the blobs of cast iron are there, just not machined). Perhaps "-037" applies to the as-machined part, but since GM never sold this in service we don't see it in the parts books. Only -036 shows up. And then that changed in '69 with the RAIV cars that have no heat riser pieces.
Wish I could see the OE drawings from back then...
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