There is a 3 digit number 9xx behind the trigger guard and where the pistol grip area is. It is also the same number stamped on the lower portion under the barrel when it is broke open. Is this the serial number possibly?
Yes, that would be Army & Navy's number. Understand that A & N didn't make it, as they were only a retailer. Most of their guns will also bear the trade maker's serial number as well, which will usually be found on the underside of the barrels (on the key rib, fore-end hanger, lower rib, etc.).
