Tommy,
The B31 is the 'Touring and Overseas' version of the A31, which itself is an updated in detail EW. According to catalog specifications, the petrol tank holds 2.5 gallon with 0.5 gallon of oil in the aluminum sump under the engine. The A31 tank hold only 2.25 gallon, but it also contains a compartment for the oil, as it has no sump under the engine. This compartment may account for the 0.25 gallon, as it is not very large.
But from catalog appearances, the B31 petrol tank looks taller and slightly larger than the A31. More akin to the other larger capacity models offered in 1931. But these all have petrol tank capacities given as 3 gallons. So it looks as the B31 petrol tank is particular to that model. It may have used the same pressings as the A31, and the difference in the catalog is just down to artistic license. Besides the lack of an oil compartment, it has a gate screwed to the side of the tank for the hand change, where the A31 has the gate built into the right-hand knee grip. The A31 was build down to the 224 lb weight class for a special road tax savings.
You will probably find that the L3 and H3, the 1930 versions of the B31, use the same petrol tank. Pictures of L3 and H3 indicate this is so, and they are the same size and shape as the A31 (except they do not hold oil.) The B32 (for 1932) shows a shift gate built into the knee grip and may not have the tapped holes for a separate gate. Again the tank looks bigger, but still lists at 2.5 gallon verses the A32 at 2.25 gallons and all the other bigger machines at 3 gallon.
I can send you a catalog scan of the A31/B31 if you would like, send and email address via the Private Message function.
-Doug
