That's a Hayes mount. Trek had them on the later Y bikes as well as many of their high end hardtails. A few other manufacturers used them as well, but they soon moved to IS.
IIrc Hayes still make adapters for them (at least they did when I last looked). Hope also made adapters for this mount too but the only calliper that would fit at the time was the C2 (I had one of these on my YSL300 which used the same mount).
Saracen Kili Ultra Titanium 1994 - SOLD
Avanti Tuatara 1992
SR Sakae Litage carbon 1988? - future build