Flathead Date Code
On late model flatheads without the bellhousing cast into the block, there is a code stamped on the right rear corner of the intake gasket face of the block which will give you the manufacturing date of your flathead Ford or Mercury. You can see it with the intake manifold and heads still on the engine.
It consists of a series of a letter, a one or two digit number, and a second letter. The first letter is the month. The number is the day, and the second letter is the last digit of the year.
The letter code is M=0, G=1, B=2, L=3, A=4, C=5, K=6, H=7, T=8, R=9, S=10, E=11, F=12.
G 20 B would be an engine manufactured on January 20, 1952.
As for the other styles of blocks, most folks can identify the engine series if not the exact date of assembly. If you can't, refer to one of the flathead books available or one of the other flathead web sites.
I should credit Flathead Jack's catalog as my first source for this information, although I have now seen it in several places.
Please remember that it is possible, even likely, that the year the engine was assembled will be the year before the car's model year. Also, I have heard, but never verified, that if Ford rebuilt an engine, they changed the date code to reflect the rebuild. I have seen flatheads on which the date code appeared to be missing, although I suspect this is an artifact of time, rust, and/or machining.
Here are four photos to help you find the code. I hope this is helpful.
The shiny reflection on this engine's surface is where the date code is located.
This is the same engine as above. The date code of S 24 M means it was assembled on October 24, 1950. It could have come from a 1951 vehicle.
This rusty hulk came off the assembly line on January 9, 1950, so it's safe to assume that it came from a 1950 vehicle.
This one was made on December 12, 1949. It came from a 1950 Mercury.