The flywheel comes in 2 versions. A 6 volt and a 12 volt. Yep, the trick is to match the flywheel and the starter. A 6 volt starter will work just fine on a 12 volt truck. Turn that ring around on the flywheel and match the correct starter to it. And you will drive away happy. There is a different tooth count and pitch to the ring. The tooth count is easy to find in "search" but here it is.Tooth count, 6 V is 139 teeth and 12 volt is 168 teeth. The starter has the same number of teeth for 6 and 12 Volt, but the pitch is very different. There is a difference in the shaft diameter. A good rebuilder will know what you have.

Steve H