What Dan said about replacement engines being restamped with the original serial number. Pretty common occurrence if the vehicle was registered with the engine serial number instead of the body tag number (VIN in later years,) I have a 1953 casting number 216 engine with NO serial number on it in my 1951, but have documentation that it was installed in the truck in 1954 by the government agency that owned the truck then. I also have a 1958 235 engine per casting number with a serial number indicating it being a 1955 engine. That one was likely installed in a 1955 truck as a replacement, as there were no time machines back then to get a 1958 engine to put in a brand new 1955 vehicle.
