Are both barrels marked as 2 3/4"? If so check the numbers, they won't match the gun.
What you have is a pre-war A5. The position of the safety is a dead giveaway. All pre-war 16's had 2 9/16" chambers.
If the barrel numbers do match, and your gun functions with 2 3/4" cartridges you are either one of the lucky ones or your gun has been altered to fire them.
"There are three kinds of men
Those who learn by reading
The few who learn by observing
The rest have to pee on the
electric fence and find out