"Twin carb" A10 heads are simply 1954-55 aluminum heads with the removable single-carb manifold.
Later A7 and A10 heads had the single carb manifold cast as a part of the head itself.

BSA's USA distributors began marketing a dual-carb conversion kit to adapt the removable-manifold-type heads
for two carburetors in about 1963 or '64.