Hi Barry, from the information I have, the following are marking definitions.
RG 2 3/4hp (EW)
RG/A EW box with a new bush and steel sleeve for low gear.
RG/B EW box with sliding gear, ball plunger, no vent pipe hole and a new operating leaver.
RG/C EW box with new anchorage fixing.
RG/D Special box with low ratio for competion.
RG/E Box with stronger heavier kickstart quadrent and pinion.
RG/F Close ratio gears.
RG/G Box with solid gear (H3).
RG/H A/31 350cc gearbox.
Roy.