used as shorthand for the units (as when one sees a weight as 20#).
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Ah, but there's an instant regional problem here:

In the UK, I have never seen a weight listed as 20#, but always as 20lb. Likewise, I have always seen # referred to as 'hash'.

I suspect that this comes back to our currency being the pound, as indicated by the '£' symbol, and that as a result it would be confusing to give the same name to anything else.