This is the Robinson Racing Super Hard "Absolute" 48P Steel Pinion Gear 23 Tooth.
Super hard Absolute steel pinions gears are a must for running with steel spur gears or will last and last if run with a plastic spur gear. Sizes 16 through 28 teeth. Tooth number stamped on each pinion. 1/8 inch bore. One 5-40 set screw included. 1/16 inch allen wrench (not included) fits the 5/40 set screw.