The 5 crown 12-gauge pump-action shotgun has a texture issue when attaching the 5 crown shotgun scope - it uses the worn finish of the lower-level receivers:
Usually shotguns don’t have a rail system by default, so welding a rail to receiver (replacing part of the gun in this case) would be the logical choice.
I guess that’s an explanation for the way it is affixed to the gun. It doesn’t explain why the 5-crown scope has a rusty base tho - it seems like an oversight.