Formula disc brake standards 97/98

[12345]

Hello,
I have an old frame with an odd disc brake mount that has IS2000 hole distance. However, I cannot mount a modern PM disc brake adapter on it, because the seat stays don't leave enough room. Originally Formula disk brakes were mounted, but I don't know which standard they had to comply. Are any adapters available that allow me to mount a modern PM disk brake on such an old frame?

 

[sam_white]

What size rear rotor are you running? You might find that a bigger rotor moves the caliper further inboard and therefore leaves more space.
I think your best bet is to try and lay hands on a couple of different brakes and test fit them. Your advantage is that you have IS mounts and rather than the obscure Gary Fisher/Hayes mount down there.