I believe one difference in the fuel pumps is the angle of the in/out ports. Early pumps, to 1951 I think, had ports parallel to the block whereas later models did not.
I believe all 47-55.1 truck pumps were originally parallel. The angled-port pumps were for cars to avoid interference with the side motor mounts.
At some point some of the aftermarket bean counters must have decided to make them all angled to simplify manufacturing and save money. Not a big deal, since you can easily bend the fuel lines to make them work.
Carl
