Those are swiss clips- the thing I'm looking for is similar in concept, but has a much bigger "clip" that shows in front of the glass. Pretty much covers the entire corner, instead of having just two little tabs showing. I know most exhibits don't want them - I don't normally want them either for a variety of reasons, but there's a show I've been asked to participate in that welcomes minimalist framing. Worst case scenario, I can just package the matted prints and the plexi and include enough L-pins that they can mount the work that way.