103s are wonderful guns. Very well made for the price. I currently have a 12 ga 103A gun. The letter after the 103 designates the grade of the gun and indicates better wood and more engraving. If the gun also has a S after the A,B,C then that indicates special nickel steel barrels. The barrels will be stamped as such (on the top) and the barrel flats will be marked with a swastika which is an old symbol for good luck, as I understand. There was a site that had good info on the Huskys but last time I checked the site was down.
Your gun should have originally had 2 1/2" chambers & I would not recommend shooting 2 3/4" shells due to the steep/sharp transition from the chambers to the forcing cones.
If I remember correctly the Double Gun Journal mentioned above is vol.9
issue 3.