The 1901 sight has a different front screw that the sight pivots around.  Believe it or not the screws are metric, a holdover from French influence in the early days of our armories.  It is an M4 screw, but the pitch is 1mm and the standard pitch for an M4 now a days is .7mm.  I've found some old japanese made tip off mounts that have an M4x1.0 screw.