Obviously those holes are for a mount that was amp intended for disk caliper. Amp research still sells spares and are located in California. Email them to see if they still have it or has a tech sheet to have one made. Otherwise one could be welded on by the right welder. I have a weld jig for the mounts and do 2 or 3 a year.
You could make your own template by tracing the hole pattern off a fork.
Also Ebay(US) has adapters that bolt to your canti bosses.
http://cgi.ebay.com/New-Bike-Rear-Disc- ... ccessories
'90 Offroad "Proflex"
'97 Proflex 957 Large, '97 Proflex 957 Small
'99 K2 OzM, "98 Proflex 4500 frameset
'00 K2 3000, '00 K2 4000
'02 Serotta CSTi, '95 McMahon FS