You do indeed have what was cataloged in the 1950's as a 20 Magnum Model 21 with the added features of Vent Rib and non-standard barrel of 28" down to the original butt plate which was standard. There are 123 of these guns according to Schwing and Winchester records. They are a seperate group from any other 20 gauge 3" Model 21's to a collector.
A truly magnificant gun! Resist the urge to install a recoil pad and leave the gun all original. Have the choke constrictions professionally measured. Get some Kent TM and go shoot some ducks! I have only seen a few of these and yours is one.-Dick
