A2Z components made adapters for those forks. Actually formula and IS mounts are the same bolt spacing as I recall, but the mount distances from the axle center is what is different. IS calipers fit formula mount tabs, but then the standard rotor size ends up a mm or so too large in radius, and the top of the disc rotor carves into your caliper as it spins. The 1998/99 Manitou forks used the formula spacing, when I ran a IS caliper on one with a 160mm rotor, it dug a groove into the top of the caliper as well as into one of the mounting bolts.