The thing to check would be to see if the Belgian Proof Marks reflect the 3-inch chambers. I can't imagine shooting the 1 3/8 ounce high velocity or 1 5/8 ounce Magnum 3-inch 12-gauge shells of the late 1930s in a Lightning!! Looking carefully through my 1938 Browning Arms Co. paper I don't see anything about 3-inch chambers. Under Standard Specifications it states "All guns chambered for 2 3/4" shells." The plain matted barrel Lightning was $75.80, solid rib $79.80 and ventilated rib $89.90 in 1938.
You might want to call Browning and ask for the historian, Glenn Jensen, and see what information he has on the gun.
