What is the forge mark... Is it a keyhole? That's Cerro, and is correct. The CMMG uppers I have, and friends have, all are Cerro forge.
Are there any other markings under the handguards? They may be very faint. The CMMG barrels I have seen and own are marked "CMMG" before the other information.
Is the receiver extension commercial? CMMG uses mil-spec... But TAPCO is normally commercial.
CMMG rifles similar to this typically have that style of buttstock to being with... So it wouldn't make sense to replace it with TAPCO.
Is the FSB "F" marked?
Oh, and does it have a piston?
Well, we already know that the buttstock isn't CMMG at the very least. If it is a build with CMMG parts, it would be OK as long as the guy did a good job.
It isn't looking good so far...