Defroster Vents

There is a problem here, somewhere. I know the vents for a 79 are different then for a 90 something. The number L716DX9, is the number my dealer got from a paper catalog, and he said it is still available. The link you posted shows that part number for 94-97 and no longer available. So I don't have any idea which is correct, at this point.
PS: I did look at the 78 truck service manual, and they look to have round connections.