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Looks like “C3” to me; the size I assume.
Edit: There’s loads of Selmer mouthpiece info here
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My knowledgeable friend says, paraphrased -
It's called a selmer ff which wasn't made by selmer at all, they've historically been quite 'nothing' mouthpieces. But recently the alto ones have become very sought after because Charlie Parker played one on him famous 'bird with strings' album.
I believe they were manufactured for a shop called Bill Lewington in London. A good condition alto could be worth between £300 - £500. Tenor ones still go for £150 or more
I don't but if you I know a man who does - if you post a photo I can send it to him and ask his opinion?