Yep, Miriam got it covered ... mostly.
I have two Singer and one Greist slant shank button hollers and none of them are marked.
The Singers are in the maroon colored plastic rectangle boxes. The Greist is in a paste board box.
I have several Singer straight shank units in the green rectangular boxes and none of them are marked either.
I have one Singer straight shank unit in the green egg shaped box that is marked "straight".
None of the Greist straight shank units are marked.
Basically if the button holler came in a plastic red, pinkish, or maroon box of any shape it is a slant shank unit.
If it came in any plastic box of any shade of green it's for a straight shank machine.
The Professional and other button hollers in the paste board boxes are harder to tell. You have to read the label on the box or look at the button holler to see what's in there.
Joe