To install IBM slide rails into a threaded hole server rack, you need to lock the bullet pegs back and use screws.
IBM Slide Rails: Bullet Pegs
The IBM Slide Rails use a complicated locking mechanism commonly referred to as Bullet Pegs. This mechanism allows the IBM Slide Rails to be quickly installed in an IBM Server Rack. An IBM Server Rack can be quickly identified by it’s use of large round non-threaded holes.
The IBM Slide Rails have a flange on the end of the rail that wraps around to the front of the rack upright. It’s this flange that allows the IBM Slide Rail to be attached to a threaded hole rack. Keep the Bullet Peg Assembly locked back and simply use screws to attach the slide rail to the rack.