It looks like a SKS U Stay Mounting Clip, part of which has broken off (or been trimmed off). They are designed only to be used with SKS Edge type mudguards and their U Stays, with the U Stay mounted inside the mudguard as per the image below. Whoever fitted the mudguards on your bike reversed the clip to fit the stay on the outside of the mudguard, possibly because the bend in the stay was not a good fit inside the curved profile of the mudguard.
You can buy replacement clips from SJS, although personally I would prefer a draw bolt for external stay mounting.
https://www.sjscycles.co.uk/mudguards/s ... ting-clip/