I've got a 1955 Dodge C-1-B6 with a NP420 4-speed. The guy who sold it to my Dad (in 1973!) said it was late in the model year and maybe got some '56 parts, but who knows. In any case, it's got a casting number C-88221 with a top cover C89988. The pictures below are from another post, but mine looks the same, with e-brake and all. (Not sure about the tower though.)
It's being rebuilt, but the synchronizer is blown and I can't find a replacement. Any ideas? I've asked D. Cirillo and Vintage Power Wagons, no luck.