Windy: That serial number is the standard form used from '57-up. This one tells that the engine was built at the Flint plant (F) on December 17 (1217), and the H suffix is for the regular production Thriftmaster 235 used in the '57 31-32-36-3800 series trucks. The block casting date code (L66) decodes to December 6, '56 in this case.
