Maybe rounded corners as opposed to rounded edge? Or maybe that is what you mean, and I think of something different when I think of rounded edge.
This is what I think of for rounded edge:
View attachment 207339
vs rounded corners:
View attachment 207341
The pictures of the ESB hardware are poor to be sure, so it might be up to interpretation. I see a straight edge like the ROTJ. Maybe they were all from the same parts bin? So the ROTJ and ESB/PP3 were maybe all the same?
btw: Also know as offset clips, canvas clips, canvas offsets, z clips, offsets, frame clips, framing clips, offset mounting clips.