I have a round 1x1 plate (from a blind by-the-pound secondhand sale) with a male stud on both sides of the plate, no female holes. One stud is hollow, the other is solid with a large "C" and a smaller "3" next to it, similar to a cubed notation (superscript). I think it might be C3, (Create, Construct, Customize), but I don't know how to find individual parts for that brand.
So what is it from, and how can I get more? These things are amazing for inverting directions!