Starter/ring gear not engaging

"Early 904".
What year? What model? What are we working on here?
The early '60s 904s used a different ring gear than mid 60s or later 904s, and therefore a different drive gear on the starter- a 9 tooth vs a 10 tooth on the later starters. There are differences in the starter and ring gear, and differences in the way the starters are wired and hooked up, and the years affected differ by US/Canadian production. I'm no slant 6 guru but I've seen this problem posted enough times to have it ingrained into a dark corner of my mind...
I believe @slantsixdan actually sells 9 tooth drive gears that fit the later 10 tooth (real Denso) mini starters.