All engines had what would ultimately become the stamp pad suffix scrawled on the side of the block in grease pencil; that was done in the first operation on the engine assembly line, where the block was upside-down and the bores were air-gaged. That marking told everyone on the line what innards and external parts to install in the block (cam, crank, rods, pistons, oil pump and pickup, pan, balancer, lifters, heads, intake, water pump, valve covers, distributor, etc.). The pad didn't get stamped until the engine was turned right-side-up and the heads went on.