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There are no casting numbers on my heads as the numerous websites have shown. I've only found "73B", a "G" and a "15". The valves are the larger 1.92" size. Are these the ECZ-G heads?
I found the casting number! It's ECZ-G. Aren't these fairly desirable heads? I like the fact that they have the bigger valves. Now I wish I knew if these were '57 or '58...
I'm rusty on date codes, but I think the 73 and backwards B is the date code, so 1957 (7) March (3) 26 (backwards B). The letter corresponds to the day of the month, excluding I and O for confusion, that's 24 days and the backwards letter indicates that the count started over, so 24 + 2 (B) is 26.
The date doesn't matter much, unless you are trying to build a vehicle to be as correct as possible and these are not correct if they are for a 56 F100.