On the S-8, the engine number is stamped on a smoothed area of block just below the head parting line, drivers side, either near the distributor or above the starter motor depending on year. Will start with a letter-- If a 53, should be in format of LXXXXXX with L indicating 53 and the next number 2-3-4 or 6 designating the size & series. Depending on factory options or running changes, there may be symbols or a prefix or suffix with the number. The engine serial number conversion chart is in the literature section.
Serial Numbers That head number is probably the casting number and not much help in this case since heads can be interchanged throughout years at will.
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