My mtb (SERIOUS Rockville), which currently has rim brakes, has these holes at the back of the frame and I am trying to understand their use. Are these for disc brakes? Are these for disc brake converters?
You’ll find similar decorative holes on the opposite side, which is an indication that they are not for disc brakes because those get mounted on the left hand side only.
The holes are mostly through quite a thin section of metal, recessed, which is probably too insubstantial to do anything useful, so no disc brakes.
The ‘Street’ model comes with mudguards and a rack ready mounted through the lower hole, which you’ll see is in a place with thicker metal and presumably the hole is threaded for this: