Canvas offset clip 6mm PH5801, zinc plated 100 pack.
Sort of 'S' shaped bits of metal with screw hole in one end and sometimes in the other.
Designed for use when fitting stretched canvas into a frame with a shallow rebate. Also useful for shadow box framing.
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.
Please note: May have one or two holes for fixing.