A and B Cylinders

The "A" or "B" simply is a factory manufacturing identification. During manufacturing, tooling wears as production runs. To make up for the wear factor they maker simply makes their "A" barrels within a certain specification, and supply pistons to give proper clearance when used with that part. The "B" barrels also get similar parts. There is no other difference between them. It might be of some advantage in some cases to mis match parts to obtain a tighter bore clearance than would be possible by matching an "A" cylinder to an "A" piston. Plated barrels often have several sizes - A, B, C, and D for instance. The specific purpose here is to be able to tighten up the piston to bore clearance. Often these parts are only .01 mm different in their diameter.