Orange started putting disc mounts on frames in late 1998 I think. My 1998 Judys had a disc mount, so I presume other manufacturers were going down the disc route round about this time too.
i have '97 Mountaincycle Moho
it didnt have disc brakes, and i think the following year of MC's had their own mount which nothing else really fitted
however on my resurrection of this frame i couldnt hel;p but go for discs with the "Brake Therapy adapter", it cost quite a bit for getting it sent from the USA but was well worth it