I am still looking into this and have found pics of a unrestored A12 with 11x3 on rear, car has 40k mileage and clip still holding rear drum on axle. Hope to hear from some other owners to see what theirs have. His and mine have same cast number on drums.

Found this on broadcast sheet codes

B28 is 11 x 2.5 brakes
B31 is HD Rear drums..
So is that standard on A12 or a One Of...