"A" means aluminium pistons. The crankshaft was balanced for the lighter aluminium pistons compared with the standard cast iron pistons, and got the A stamped to distinguish the difference when getting replacement parts. It started with engine No. CGP 445 A, and the latest I have records on is EGP 4331 A. Only a few engines were equipped like that but the reason why is a bit in the dark, maybe supply problems, factory special tuned or just custom ordered?