The holes for the water pumps in the two blocks are different.
The water pump is mounted entirely outside the block on the earlier block (the one with the water pump that goes above the base of the head - two small holes in the block). The lower water pump has its pump-vanes inside the block (one big hole)
If you look at the link I posted above, the locations of date casting codes (on the head and on the block) are shown.
