I have the gearbox out of the racer that had the JG motor (one of five '72 A75V 5 Speeds built in '71). The high gear does not have a part number. This uses an R&M FB148567 roller which is not the same as the Hoffman used on the early five speeds. The land where the rollers run is 1.560" diameter compared to 1.5075" for the 57-4372 high gear. The spline for the sprocket is longer by 0.107". The roller land is 0.310" wide compared to 0.332" for the 57-4372. The dogs are narrower and shorter radially. For what it's worth.
My Hurricane engine has a close ratio four speed now but was originally fitted with a five speed. Looking at the part numbers on the four speed gears in my box (pun) none of the part numbers match what is in the 1971 A75 parts manual 00-5723 FWIW... nothing to do with roller bearings used of course