What are these holes for in my Diamondback Sorrento? The holes are threaded and 20mm apart. There are identical holes on the other side where the gears are. Can I install disc brake calipers through these holes?
They are rack and fender mounts and you cannot install disc brakes with them.

I.S. mounts are 51 mm apart, and post mounts are 74 mm apart (and look totally different). For more details on these mounts, see this answer.