Starter keeps binding on ring gear, why?

Although the engine may always stop at the same two places, one of those is what you see when you take the starter off. And if there is ring gear damage you will see that spot when you remove the starter.

I've run several M's for lot's of years and had several bad starter drives (Bendix) but never a bad ring gear. I have ring gears which are very worn but still work just fine with a good starter drive.

If you should want to replace your ring gear, they are reversible. If it has been used hard for 40 years, reversing it will last 40 or more unless it has broken or bent teeth.

I have seen others throw away perfectly good ring gears because the teeth were well worn. The advice to replace the starter drive when you replace the ring gear prevents you from knowing the problem was only the starter drive to begin with.