The 'spline' is a 3/4 drive deep socket, the engage and disengage is the 3/4 drive end of the deep socket. There is a brass bushing inside the deep socket and a hole in the bolt head for the alignment rod. The spring is captive here for welding. The second pic is assembled, locked, the third is unlocked. The cable pulls the socket along 3/4" against the spring pressure, enough to disengage the 3/4 drive and the lock button on the bike brake lever holds it there. The rod keeps everything aligned. The cable isn't installed in the bench pics.
It is in the others.
