Decode engine block numbers?

[87Bob]

I have a block I am trying to confirm a decode on it can any one help? It is a four bolt main block 350 and it has the following stamped numbers:

F0413CKB & 12S253152



I get this from the books I can find:


F=Flint
04=April
13=13th day of month
C=?
K=?
B=?


1=Chevrolet
2=1972
S=St. Louis
253152=The VIN number


The CKB code decodes as 1972 350CID 165HP 2BBl going into an AB car line. The "S" decodes as a St. Louis built car which as I understand it was only the Corvette plant which built only 350 4 barrel engines. So what am I missing?