The heads NEED to match, or the engine will not run right. If the exhaust valve seat was the only difference it might not matter, but if the combustion chambers are not the same size on both heads you will have different compression ratios on each side!

I would suspect that the seller either is mixed up, or at some point a head was changed using a junkyard replacement part. A 'close enough' solution was tried.

I don't have a chevy casting reference handy, but have seen them on the net in places...didn't bookmark them but searching on chevy head or something similar should get you what you need. Used this method to see what a used 350 we bought was made of. (67 350 block, camelback/hump heads - we dont need them, we are going aluminum E-tec heads - not sure if the seats have been updated to hardened seats)