"one pound sterling," 1680s, British slang, possibly from quid "that which is" (c.1600, see quiddity), as used in quid pro quo (q.v.)
Of this usage OED1 says, in part,
[Of obscure origin] 1, A sovereign; a guinea. (Pl. usually without -s, as two quid, a few quid, etc.). 1688 SHADWELL Sqr. Alsatia III. i, Let me equip thee with a Quid.
In previous questions where quid is prominent (1) or peripheral (2, 3) I've seen nothing about the origin of the term. In a comment to my previous question, MετάEd asserts that “Quid" (money) is widely believed to derive from "quid pro quo" anyway.” But what evidence is available about the origin of quid in its sense of a sovereign or guinea?