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[On a cap of maintenance] an unicorn couchant. That's what I figure describes this crest. I'm hopeful someone can further characterize it and perhaps suggest a family or individual to fit. Note the 1843 date of assay.
Thanks all!

Names I've found associated with 'an Unicorn couchant' - Wilmot, Clayton, Rest, Donne (Doon), and 'an unicorn couchant, tail erect argent' – Doom or Bonne.

From an 1830 book: Smale or Smalley, {Paddington, Midd.} sable, a cross, voided, between four crescents, argent - Crest, on a chapeau gules turned up ermine a unicorn, couchant, (another, sejant) argent.

This appears to be the best fit I've found, but where is Paddington, Midd.?

Paddington Middlesex is now Paddington London. The old county of Middlesex has now disappeared as an administrative entity and has mostly become part of London.

Thank you for that - I suspected, but wasn't sure. The only remaining question concerns who specifically was entitled to this crest circa 1843.

Could this be the personage? - Smale, Sir John (1805-1882) Knight, Chief Justice of Hong Kong

Maybe a dead end - he was knighted by patent in 1874.

Hope I'm not beating-a-dead-unicorn, but I have 2 more questions concerning this piece.

1. Could the lifted front leg position be interpreted as lodged rather than couchant? Can a unicorn be lodged?

2. For some reason, I have the idea that the double thread pattern is military in nature - can anyone confirm or deny that idea?

Oops, found answers to my questions. The double thread is simply called military thread. As for heraldry attitudes, from the references I've read variation in presentation is an integral component of the art form.