The fact the carbine looks like it was done in somebody’s garage with the lights off should be your first clue as to who didn't do it. The second clue would be all the parts that Underwood never used.

Underwood had thousands of receivers without serial numbers on them so why would they need to grind off a serial number to make a presentation gun?

The guns Underwood did have fine workmanship and don't look butchered. They also have all Underwood parts.