The end stub of the distributor is round as that’s the part that resides in the block. The ’55 and up distributors do have a hex hole at the bottom of the shaft to accommodate the oil pump drive but you have to look at the very bottom of the shaft to see this. As Daniel brings up, the ’54 distributors have a tang at the bottom of the distributor to use the ‘slotted’ distributor drives for that model year only.