I agree with eightbore and wouldn't open the 1 gun to 2 3/4. I wouldn't spend the expense to get the 2 3/4" gun reproved. However, I would have the 2 3/4" gun inspected for safety.

If you have records from Horsley, I wouldn't engrave a 1 on the first gun.

How much change do you need to make to the stocks?

Ken