I have a Francoote sxs which has two different serial numbers on its only set of barrels. Left barrel has a 18000 number and the other a 19000 number. This is a gun in original condition. I checked the Double Gun serial number listing and it shows 1891 for the smaller number.

Any ideas why there would be two numbers on a set of barrels?

The 18000 number is represented everywhere else on the gun.