I don't know what the following means, but at least it shows there's two different pumps (taken from some 1970 water pump catalog):
GMC truck 60-67 V6 models 4000, B-4000 (w/ air brakes) & V-12 thru 9000: WP-575
GMC truck 60-67 V6 305 models 1000 thru 4000, B-4000, L-4000, S-4000 (w/ hydra.brakes): WP-1375
