‘S’ shaped steel clips with screw hole in one end and sometimes in the other.
For use when fitting stretched canvas into a frame with a shallow rebate. Also useful for shadow box framing and for assembling L-style frames.
Many framers have found that they work well the other way round, to hold the sandwich in a frame when it is less deep than the rebate.
Stick a piece of felt pad under lip for a professional finish. Zinc plated.