My '54 GMC with Hydramatic has a relay on the firewall that is not found on the manual trucks. It has something do do with the starting circuit. That relay is another difference and unique part and the only one I can add to the list from memory.
I guess as a Hydramatic owner I don't find them real rare. I know a guy who has a '54 Chevy Hydramatic truck and another guy who is a member here who has a '51 Olds 88 with a Hydramatic. Those just the ones I think of real fast and are just guys I know around here.
