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.
