this post assumes the "gear" being referred to is the "turtle" gear, if not, please disregard.
IIRC, when removing "the turtle" a few months ago while replacing my speedometer cable, i noticed it had the number "18" cast into its housing. it referred to the number of teeth in its gear. it's my understanding that some may have had fewer or a greater number of teeth. new and used ones are available. thinking i needed a new one, i bought a new "remanufactured" one online that turned out (actually it DIDN'T TURN which was the problem) to be defective...win some, lose some, eh?
removing "the turtle" (i've got a '41 ford super deluxe, BTW) is not difficult, but one of the bolts IS difficult to get to. ...just takes time.
if you need a new one, and if you can get yours off, count the teeth and buy accordingly. calculating what it should be involves a lot of variables, many of which are indeterminable, IMO, on such an old vehicle.