Ok- so the divot in the first picture does not matter, but I've noticed that the some of the singer shuttles had it and the older did not - but don't know if it really served a purpose or not. I think it was the newer ones that have it. But maybe that was something the companies that used the same shuttle as Singer came up with to make theirs different. I should check my Boye shuttles and see how they compare. I have machines by Singer, Free and White that take that style/size shuttle.
At least it's real easy to identify - the only one with the "waist".