There are two types of barrels used on the Ithaca-SKB 300 and 900 recoil operated shotguns.
The first way to tell the two barrel types apart is that the type 1 barrels use a solid bar style ejector and type 2 barrels have a split claw style ejector.
There will also be a slightly different shape in the barrel extensions as well.
The barrel extension on the right is a type 1. Notice the solid ejector and shape of the extension. The type 2 on the left shows the claw style ejector.