I'd agree with Julian. It's a plunger gear. The next layshaft gear (23 teeth) is also different on Plunger and S/A boxes. I assume the drive dog sockets differ to accommodate the different profile of the drive dogs, but in practice there seems little to tell these 23 teeth gears apart.
Plunger and S/A boxes share layshafts with the same dimensions, but as Julian notes, circlip groove position is different to match the internal profile of the appropriate gear. S/A layshaft has a scroll at the closed bush end. Plunger shaft is plain.
The circlip is there to locate the gear in production. In the shed with time to play you can press on the gear with care and adjust the shaft end float by its position. So any pair of of these two gears will work on any shaft without the circlip.
Layshaft and gear have an oil way drilling, so line up the hole on the spline and press the gear on. Worth checking the holes still line up, They may require attention. Layshaft is hollow, so give it a good clean out along with the other oil holes while you're there.
Previous posts would indicate a lot of gearbox noise in third gear is down to this gearbox pinion, and this is also when the blind bush is under greatest load. Drag's gear is certainly a choice for the well heeled, but their online parts listing conveniently shows what parts are common.
For those folks with a pile of bits, building a box up from various mix'n match parts is straightforward...bearings and the seal are the same across the range. Early Longstroke box has slightly different gear profiles so will not mix, but apart from that fixed layshaft pinion and its 3rd gear partner, all other gears are the same on a standard box. So late S/A gear clusters and layshaft will fit an early Rigid/Plunger casing. Kickstart quadrants interchange, but must be used with the matching ratchet pinion, plenty on the Forum about flat v/s pointy teeth.
Swarfy.
