Real McCoy (1914, 1915, 1916)

[Jonathon Green]

So far, my own early cites are these

1885 _Bulletin_ (Sydney) 21 Mar. 62: A Melbourne paper says that ‘a
subscription is being raised in Hobart to remunerate Mr. Matthew Burnett for
his self-sacrificing work in the cause of temperance.’ And very proper too.
We shall be proud to contribute a bottle of the real old Mackay (kept
specially for the regalement of temperance lecturers who do us the honor to
call on us) to the fund. [...] He should not be expected to get thoroughly
sober for the rest of his life

This is obviously a simple liquor reference; HDAS has earlier ones.

1901 _Boy's Own Paper_ 4 May 484 Giving it a good ring to show it was the
real McCoy

This is undoubtedly the boxer's spelling. And while I don't have the context
to mind, it would seem highly unlikely, given the moral stance of the BOP (a
British publication, very much of the 'muscular Christianity' persuasion),
that 'it' would be alcohol (nor would one give liquor 'a good ring.')..

1903 Furphy _Such is Life_ 211</num>[This] made us feel the station
aristocracy to be a mere bourgeoisie, and ourselves the real Mackay.

As Partridge notes in DSUE8, Australia maintained the 'Mackay' spelling in
non-liquor contexts..

BTW. Just to complicate matters, with an apparent blend of both spellings:

1947 M.H. Boulware Jive and Slang n.p. Real McKoy ... Just the thing

Jonathon Green