Per Pete McCarthy's book: all 1960 production engines used 536109 head castings (there were also 535461 castings for SD heads sold over the counter.) However, heads on 10.75:1 high compression production engines had an 'X' stamped just above and left of the casting number. 425A Trophy heads with HD valve springs had an 'O' stamped above and right of the casting number.
This method was used from 1958 through 1962.
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