What are these marking?

On the small blocks they had A B and C pistons that were all considered standard bore but slightly different in size to fit the bore variations . Today with computer controled machinery there are no variations . It was the same for many motorcycle manufactures back then . On my 2 stroke Yamahas you have a .97mm .98mm .99mm variation on all standard bore sizes .So,those letters could have been how they were marked before assembly .