Possibly a job number applied during a rebuild of the heads or engine at some point.
You don't say where the car where the engine came from was built but there were some workers at at least one plant that sometimes used the head rather than the block to locate the VIN. Problem with that is there are too many digits and body code of 53 would be a Ford Custom 500 4 door sedan. Guess that is a long shot since you don't report what kind of heads (engine size) or casting numbers they have
Guessing at best since you didn't provide much information to go on