This is a double ARCA Male to Male adapter. This is great for attaching to a tripod and an ARCA clamp on a plate, creating like a Tactical Table for shooting off, use to rest binoculars, small camp table, etc.
it’s about 3% oversize to allow clamping with either a anvil or screw down clamps, of all the rails I measure, there is no consistency across them so I slightly upsized it