A picture of the rear stays would be useful, to see where/if the canti bosses have been removed - surely there'd be some evidence there? The rack mounts, position of the gear cable stops and brake cable stops suggest it's an MTB frame. Nitanium MTBs (Rockhoppers and Stumpers) used a 30.4mm seatpost if that helps. It does look for all the world like an MTB frame, but why would anyone remove the brake bosses unless they were going to add a disc tab?
The seatstay bridge on my 98 Stumpy doesn't have a hole, it's tapered towards the centre from both sides, the Rockhoppers used a cheaper straight tube , can't remember if they're drilled or not .