Don't know if the powers that be have changed it but it use to be the first letter was the decade the second was the specific year of that decade. C=60's D=70's E=80's so a casting number that starts C5 would be manufactured in 1965 and one that starts with D5 would be 1975 ... the heads could be identical (made on the same mold pattern even) but different casting numbers because they were made at different times.
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