CD-you are right, I can never keep that straight. So newer machines can use older shuttles, but older machines can not use newer shuttles. Though I bet one could swap carriers on an old machine so it would not need the shuttle with the notch. Though both styles of shuttles are pretty easy to get.
CD-do you know when the switch happened? I was researching about a VS2 fiddlebase (one machine I'm still looking for) and read that it needs the notch.