Those most likely engine "serial numbers" for 235 or 261 blocks from 1957-1963 (when that code format was used)
F = Flint, MI engine plant
203 = date code (month then date) = Feb 3rd
JC = truck mode/equipment code = might give year indication, too, depending on the year
= 4000 series (and/or RPO 227 heavy duty clutch)
F = Flint, MI engine plant
1030 = date code (month then date) = Oct 30th
XTA = truck model/equipment code - might give year indication, too, depending on the year
= 4000 series (and/or RPO 227 heavy duty clutch)
That format contains no year information and no "serial/sequence" number.
This information was extracted from GM Specifications Manuals)
