This blade-type drive dog is a replacement for the Smith's input end of a mechanical tachometer cable. These little dogs tend to go missing when you disconnect the cable from the tach drive gearbox, which will leave you without a tach. Carrying a spare is cheap insurance. Sold individually, one required per cable.
This part is also required to complete our Smith's to Smith's Tach Drive Cable (Part No. 1382, sold separately).