I've sailed and re-rigged small full scale yachts and catamarans all my life and never seen a fitting like that. Perhaps it was put there as a halyard or stay 'tidy' for storage purposes? looks more like an upside down boat cover tie down clip or part of a latch. Perhaps try the door and lock section at your local hardware super store. Found this on google. As the other guys have said though, probably hand made. Usually, no rigging fittings on yachts have square edges unless they are part of a fitting.

Thanks for the ideas guys. I'll probably try and find a two horn cleat like you suggested, though finding one I can blind screw in at two points is proving a little more difficult. Like you said, it's looking like a handmade fixture. I suspect the whole boat, including all the fixtures were hand made. The joys of buying second hand.

I'm in the process of researching running and standing rigging for a boat of mine. I've not seen one of those anywhere so I suspect it is hand made.

Closest i've seen.
https://www.cornwallmodelboats.co.uk/cgi-bin/sh000001.pl?WD=fitting%20forestay&PN=CAP%2dMaquettes%2dMetal%2dFitting%2dMast%2d23mm%2dCMG400%2ehtml#SID=2131

Looks like a simple 'Jam cleat', roughly half a normal Horn cleat.
On a mast a form of Shroud cleat.
Look in the Fittings section of any good model shop, e.g. Cornwall Model Boats. if all else fails just buy a normal two horn cleat and saw/file one horn off. Simple enough to make though.
😎

Thanks, that's great, but doesn't answer the question at all. Your lovely diagram fails to note the metal hook above the spreader on my photo and it's that that I would like to know the technical name of and where to buy them.

Hope this helps I no expert but wikipedia explains it.

Standing rigging on a fore-and-aft rigged sailboat.
Key: 1. Forestay 2. Shroud 3. (Spreaders) 4. Backstay 5. inner forestay 6. Sidestay 7. (Boom) 8. Running backstays