There are several possible explanations, any or all of which are common.
It could be a replacement block.
It could be that somebody added a water pump.
For instance, I own a 1940 B with the original C-113 block, matching serial numbers and everything. I added a Super C water pump on it about 7 or 8 years ago when I overhauled the engine and installed 3-1/8" diameter sleeves. That is a very easy and common upgrade on these engines.