"ABL" means that O.S uses two metals to plate the brass liner. A flash coat of an under-metal, then the nickel plating that the piston rides on. It's a newer process than they used to use when they just plated the nickel directly onto the brass. The "bimetallic" refers to the fact that two metals are plated onto the liner. This process insures better adhesion of the plating to the brass liner.