The B52 gearbox used by AMC is essentially a GB gearbox as used by Ariel but with a different mainshaft and layshaft (no speedo drive) and end covers. The GB gearbox uses the same size bearing as a CP.
I don't know about the Numbers they are identified by but the AJS/Matchless box is essentially a C box with different outer cover and mount lugs. The CP main bearing is apparently not currently produced. A modified similar "imperial" sized bearing is adapted by grinding the inner size a bit larger to fit the sleeve gear. This is what makes the bearing more expensive. Nev