Final drive sprocket for a solo is 21 teeth.
The selector fork has a “V” cut into it and the selector arm has a corresponding male “V” that engages with the selector arm. On the rear wheel side is a plain bush and the whole assembly is clamped together by the bolt with the slotted head. On one of my gearboxes, the slotted bolt has been replaced by a normal hex bolt and a washer to retain the plain steel bush. The thread is right hand.