What's The Deal With The Mopar Mini Starters?

Ok, I THOUGHT ( dangerous, I know) the Mopar mini starter such as for a 92 Dakota worked on all things Mopar. I found out the other day they do not. My friend Matt has a 68 Dodge D100 2WD, 318, 727. His starter began intermittently not cranking. So he pulled it in his shop and ordered another one. Store sent him the mini starter. He stuck it on. It spun just fine but the bendix did not throw the gear out far enough to engage the ring gear on the converter. He pulled it off and noticed not only did the gear have about a 1/2" long "nub" on the end, but also when pried out, it did not travel all the way to the end of the inside of the nose. The old big style's gear was flat on the end and it did travel all the way to the end of the inside of the nose. Both starters were the exact same dimensions from the bolt bosses to the end of the nose. Matt called me to see if I had a good old style big started and of course I did and he put that on and it's worked fine. So what's the deal?