Which would raise another question. Why would a serial number go only on the part of the gun that is NOT essential to making the gun shoot?
Good question. The lower section of the AR15 type gun is essentially a trigger group. The serial number is not on the trigger group of the Garand. The receiver part of the Garand does have the serial number. It does seem like the similar part on the AR15 should have the serial number. My guess is that the BATF agent that gave Stoner the serial number for his first AR15 prototype was stoned.