This is the flat part of the metal frame where the barrels rest when the gun is closed. You must "break" the action or remove the barrels to see these stampings.
While the barrel held the primary serial number, manufacturers often stamped a matching number on the receiver to ensure the parts stayed together during the manufacturing process. Neumann Bros Shotgun Serial Number Location
Once the barrels are removed, look at the "flats"—the flat metal surfaces on the underside of the barrel assembly that mate with the receiver. This is the flat part of the metal
Here’s a focused feature on the , including context, tips, and variations. Neumann Bros Shotgun Serial Number Location