There was a fella that made the 110, and 112 auto conversions for Cumberland Mountain that made them with that square to the bolster push button. I recall this because I was trying to find the best 110 auto and there are a LOT of them out there. I personally prefer the oval button so It's the button style, and placement that sets most of them apart.
I've been looking for what I had read about them, and the best I could find for now was a google image of a rare left side button but it is one of the Cumberlands with their etch on the blade. 1990's maybe?
The earliest ones had just a stamp on the pile side tang... CMKT
Use the Buck tang charts to date your knife, and that will help narrow it down.
Those early Cumberland knives have some value to them so I would keep chasing the leads until you can verify where yours came from.
I hope yours is one of the good ones.
WTB Kershaw 2120 MACHO Lockback Parts knife
I need a pile side scale. THX!