2003 Trek Elite 9.8 rear disc help



Hi,
I bought a used frame and want to instal rear disc on it.
It has 2 mounting holes on the rear triangle.
I guess those are for the caliper.
They are 6mm diameter / ~45 mm apart on opposite sides of the hub.
Does anyone know what adapter / caliper fit in there if at all?
Advice appreciated.
Thanks.
NH.