I looked at drawings for 731s and 752s a few years ago, just to see if 752s would go under an RS-1 in it's existing trucks. Afraid not, the 752 is perhaps two inches longer betwen the axle centerline and support lugs, up against the bolster. (Which has made me wonder how people get 752s into Blombergs, but that is another story-).
The RS-1 actually used two different trucks, very similar looking, but the parts are not interchangable. The change occured around 1950, and seems to have coincided with the change of switcher trucks, from the Blunts (S-1, S-2) with 7x14 bearings to the equalizer truck (S-3, S-4) with 6 1/2 by 12 bearings.