A hand driven massage machine looks very similar - but the business end is about 2" diameter - some old valves also had two holes, rather than a slot, so a forked end attachment was also available. Never used a massage version, but I would guess it has some axial 'hammer' action, whereas the valve grinder reciprocates a half turn to and fro (on the better ones not 50%/50%, but 60%/40% so the valve gradually rotates as it is being ground in)...