I should add that the clip goes on one way on some units.
Really? I never noticed that they were asymmetric.
I hate these things. Hate them. Spent the better part of an hour yesterday struggling with one. Not because it wouldn't clip in, it would do that just fine, but because it wouldn't hold a decoder down with enough force to make contact with the brass rails. Grr. (Turns out the problem was not the clip, but the decoder design, which, for the given loco, didn't have cutouts for the little gray posts that hold the brass rails in place, so the clip was pressing the decoder against those posts, which interfered with the fit. Still, I hate the whole design.)