What make is the gun? Who is the manufacturer ? "mint" is a relative term. It has different meanings for different people , and isn't meant for describing a gun's condition
because of that . It is used in describing such things as coins , that are uncirculated , sealed in plastic , and never been touched , or had a fingerprint on them.
Sounds like you have a beautiful old gun there , in great shape. But is it a Fox , an Ithaca , or a Stevens ? European maybe? They are like apples and oranges.
"I have an old 16 side by side which is mint" , isn't a good enough description for you to get a knowledgeable answer.
Call Gunco , Jason Spencer , Reg'd Gunsmith in Ottawa. He's the man you want to talk to.
Have a all your information ready on a scratch pad before you call. Make / model / barrel lengths / etc. He will want to know what it is too.