I've just got a new keyboard at work and haven't had it de-Americanized yet, so I have to remember that @ makes " and all that.

This set me wondering: where on earth did the hash # key come from? It had no use in typewriter days, did it? Presumably its use in computing is because it was there available on existing keyboards? But why? What was it? I've never seen it used in print in a pre-computing context.

(Yes, of course it can be colloquially read "number" as in #1, and legend has it it's an American pound sign, but I've only ever seen 2 lb, not 2#. Surely neither of these was common enough to get it on typewriter keyboards?)