What is this letter/symbol called?

I can't follow that link to anything useful, but pending the addition of a picture to the question I'll provisionally suggest that what you describe sounds like it is could just about be the letter yogh (capital Ȝ, lowercase ȝ), but more likely the letter ezh (capital Ʒ, lowercase ʒ).

ʒ was invented for a system of English phonetics, and survives in the IPA.

There was also an old scribal abbreviation for the unit dram/drachm that was so close to ʒ in appearance that the symbols are essentially merged at least as far as modern computing use goes (though some use ȝ rather than ʒ there).