Mossberg Model 500A Serial Number Location - ATF Compliance

Question
I AM TRYING TO LOCATE THE SERIAL NUMBER ON A MOSSBERG MODEL 500A (NOT PRE 1968)

Answer
On shotguns, the "gun" is the receiver according to ATF - everything else (barrel, magazine, stock, bolt) is just "parts."

So the SN will be on the receiver.

I have never seen SNs anywhere on a Mossberg other than on the left (non-ejection port) side of the gun near the elevator / magazine access.

ATF stipulates that SNs must be readily observable, so it cannot be covered up.

If it is not in this location, it is possible that you have a proprietary receiver, those these are very rare.