Who is the manufacturer of the USGI Aimpoint mounts? What is the quality of them as compared to the ARMS, GG&G and other current offerings?

think they are using the aimpoint mounts.

Aimpoint QRP:

The QRP is what you will usually see as it is the basic issue for the M68CCO.  HSLD types will have a variety of mounts from ARMS, KAC, LaRue, etc.  Some line units will allow individual purchase optics and mounts like the previous listed.
In short, there isn't a universal answer.
ETA:  Their quality is good, but the other mfrs figured a better way to do the same thing with less material.  The QRP is solid and has good repeatability, but it is rather long and that torque controlling knob sticks out quite a bit and has a tendency to snag stuff.  The ARMS rings let you remove the optic with good repeatability and it takes up a lot less rail.  The throw lever folds flat against the rail.  The GG&G units have a nice spare battery cover and storage.  The cantilever mounts allow you to push the optic forward if you don't have a FF RAS system, or don't have a RAS system at all, or if you need to mount other optics on the rail.