The Singer button holler cams are the same as the Greist button holler cams. I have many sets of 4 and a set of 10. What they are I cannot say as the box they're in is still in the storage locker.
They were originally made from metal then later plastic, so an adventurous individual might could create all sorts of cams.
But .... from my collection of button hollers, I'm thinking there may be more than 10. If I remember I'll check them out when I get the stuff here.
Joe