In your last photo, you can see the small drain hole and passageway which have to be filled in to mount this style block on the early mid sections. I’m not positive but I think the tiny hole and passage started with the thermostat motors, ’59 and later, It’s probably a combination drain and an aid to help the water circulation to the cylinders. The six horse have it for sure.
You can double check to make sure there won’t be any leaks by placing your new 303467 gasket on the block to visually see that everything is water tight.
