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I've got the correct cylinder head casting numbers on my March build '66. Both heads have casting # 3873858 and both heads heads have a cast date of A256,
but what do the GM1T and GM2T mean.
They're mold numbers and indicate mold #1 at the Tonawanda foundry for this casting number and mold #2 at the Tonawanda foundry for this casting number. It's VERY common for these numbers to be different on the same engine. In fact, the real rarity is if they happen to be the same.
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