That's a damn good point- no and no is my answer.
No, in that the stowaway key is still unidentified (but under close observation).
And an additional 'no', because I've just received (yesterday) delivery of a set of keys I'd experimentally-ordered from Amazon. This was as a result of me doing a Google Images search at pictures of keys.
[By way of a quick informative aside: my dear old Dad was an extremely-talented carpenter & joiner. Something I always did with him (from a crawling and not-yet-talking age) is searching for specific screws/nails/half-hinges/keys/escutcheons out of vast repurposed biscuit tins and adopted jars, either in-situ, or spread across huge broadsheet newspapers upon the floor. I've absolutely no doubt this has helped me effectively recognise patterns, hazards and links of every sort throughout my whole life, and perhaps also created a stoic stamina within me].
Anyway, I saw these comparator keys....hands-up who recognises them?
View attachment 64188
My recognition 'click' was made visually with the three small padlock keys in the middle, especially with the awkwardly-angled one.
They are
Fire Brigade Master Keys and are apparently part of the standard pocket repertoire of facilities management or janitorial staff (and also....presumably, firefighters) to gain access to locked premises.
I had a quick bemused read of references to London Fire Brigade-approved padlocks, and the fact that these Master Keys have some quasi-universal number series (eg 'FB11') stamped on them.
I don't think I'm the last person in the world to find-out there's such a crazy thing in the world as a set of universal (well, Britain & Ireland?) old-school passkeys possessed by firefighters and professional criminals with Amazon Prime accounts: did YOU know about this weirdness??
TLDR: my mystery key looks very (very) like an 'FB padlock passkey'. And it's the same style/profile as one of the ones depicted above. But it's not precisely identical, nor does it have an FB number stamped on it. So rather than being further forward, maybe I'm slightly further away from a solution.
ps I am of course now faced with a minor moral quandary- do I now do the decent thing with the cheap FB passkeys I bought (ie drop them down the nearest storm-drain) or keep them in the back of one of the many many drawers-full of potentially-useful things I possess? Decisions, decisions....