How they're machined makes normal disc brakes not fit. When I say normal, it seems like they're all are now pm with an IS adapter, and the design of adapter doesn't fit as the holes don't extend out.
Example of the adapter shown in the link for Avid BB7 brakes...
Could be stolen, and the parts replaced. Or what I call a younger brother build. It's been moved on to the next family member who doesn't have the connection to originally buying, so parts have been replace with just basic crap.
Problem Solvers
https://www.universalcycles.com/shopping/product_details.php?id=6361&attribute=13737&gclid=Cj0KCQiA6vaqBhCbARIsACF9M6kQ5FqrQ7mdx8mANtXG1qZwa7QUrod_BP28svAtYzKxS4amU7KBbOYaAg0XEALw_wcB&fbclid=IwAR3iqeexITV1yuZKAIc2wVoN_3lJrCkcKHMG7f-SYnDfWJCZJWytK6OJBBk
A quite good condition GT Arrowhead from about 2001 - but missing a seat.
A Saracen Xcile with Hayes Nine brakes, but quite stiff, and overall a but rusty, frame looks OK.