I imagine were talking the rear wheel drive speedo device that fits onto the hub.......however the expensive bit is the girder fork type right angle drive ,thats driven by a gear screwed (LH) onto the hub.........very similar to the tacho drive fitted to the timing case of Velos and other performance bikes ........if you have any old diesels in the back yard,before scrapping them,check for one of the brass right angle drives .....Gardner LWs for instance have one hiding between the flywheel plate and the back of the block.....Perks often have a the later alloy /potmetal rt angle drive in the timing cover ......I remember back in the 70s ,the rt angle drive as a new spare was quite expensive......now I see sellers asking $200 for ally ones and $500 for the prewar brass ones.