one of these
Hi, just built one of these up for a mate.
the holes are for an old Hayes rear caliper that is no longer in production but the A2Z mount will convert this to international standard ,HOWEVER it will require the disc upgrading to one 20mm-25mm larger .
you'll also need quite a few shims as it leaves quite a large gap between mount and caliper , I custom made a pair of spacers from the large spacers from v brake pads (the one that alter pad angle)
The only problem here is that only a caliper designed for a 140mm-145mm rotor (needing a 160mm-165mm once A2Z adaptor is fitted)will fit as anything bigger would foul the seatstay and not fit in the gap.