What is commonly called the A model is the first 3406, these came into production around 1976 and stayed until around 1984 they don't actually have a letter in their identification, they just became refered to as A models when the. B came out in 84.
The B has a narrow bearing crank and what they call a single scroll injection pump, which has to do with the design of the barrels and plungers. Sometime during the building of the 3zj serial prefix cat decided they would change the crank and the injection pump, the crank is now the same as what is used in the E models when this happens and the injection pump is switched to a dual scroll style pump allowing for higher injection pressure. They offered the C model up until around 1997 depending on the truck manufacturer.
There are 4 different PEEC serial number prefixes 8TC, 5YG, 2EK, and 4CK. The first is the only B version as far as I know, the 8TC and 4CK are the most common to run across.
The A model prefix is 92U, there are two true. B model prefixes and they are 7FB and 4MG, the earliest of the 3ZJ engines are considered B models but a crank check is still reccomended before buying bearings. The later C models were 5KJ and the last was 8PN.
The E model is an over head cam engine with no injection pump it is fully electronic, it has unit injectors that are driven off the cam and controlled electronically. Nothing but the crank and rods carried over from the C models that came before it