50-53 Mercury Flahead ID

[GWRAALSTAD]

I'm trying to ID the Mercury flathead that I just purchased. I can't find anything/table that helps me ID the castings #'s on the block. The only markings that I can find are the 1BA markings in the valley of the block. A Vanpelt table says that it might be a 51 Ford, 100HP, passenger car. The engine was pulled out of a panel truck. A rebuild tag on it says 4/10/10. It has EAC heads. this tells me that it was a passenger car block beefed up with EAC heads to put in the panel truck. Any help in IDing this block will be greatly appreciated.