2003 Trek Elite 9.8 Discs?

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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 / ~44 mm apart on opposite sides of the hub.
Does anyone know what adapter / caliper fit in there if at all?
Advice appreciated.
Thanks.
NH.