Pulled this from my stripper clip thread from last year to add it into this one.
Most are 3Tk which can be solo, in a diamond, in a square, or in a circle. Some also have U, P, I, or C stamped on them and then there are 31, (31), 23, 13, 58, and 141 stamped clips as well. I also have a bunch of 386 stamped Chinese clips.