If it would fit a fork leg it could be a spedo cable guide or altenatively a universal mirror mount - big hole for the handle bar, small hole for the mirror stem.
Or the front headlight case for Action mans GS1200
That's unbelievable I saw it's pair today dragged up in
beam trawl but this one had a note attached (waterproof) saying this is a device measuring how big a **** n bull story can slip through an anglers net. Weird.