Hmmm... no one has replied for 2 weeks. I'll chime in.
The 2 starters look essentially the same in appearance... the difference is inside. The 6v 9 tooth gear rides on a 5/8 diameter armature shaft and the 9 tooth 12v armature shaft is a smaller 1/2 inch. Also, the 6v starter has 2 field coils where the 12v starter has 4. Take the band off the rear of the starter... 2 brushes is 6v. 4 brushes is 12v. 1 brush for each field coil.
The foot-start switch for a 12v starter will work just fine on a 6v starter but not the reverse. The 12v switch has a side terminal that is simply ignored in a 6v application. Hope this helps.
