Origin of “quid” in its sense of a sovereign or guinea

Question

What is the etymological origin of quid in its sense of a sovereign or guinea? While preparing the question Origin of “not for quids” phrase I noticed that etymonline's quid entry merely says

"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?

Answer 1

The OED is less confident than some other sources:

Origin uncertain; perhaps [from] classical Latin quid ‘what', reinterpreted within English to refer to (monetary) means or wherewithal. (My emphasis.)

Answer 2

According to Milroy J. and Milroy. L, "Authority in Language" (1985),

[...] quid pro quo probably gave rise to the slang "quid", a unit of money which varies with the context in which it is used.

And the plural is often the same as the singular. (same reference)

Answer 3

My mother told me that another use for quid was as a term for a lump of phlegm coughed up. That it was green and, so she seemed to infer, squarish, connected it it to a pond note. Which came first, though, though, I don't know.