Who is McCoy and what's the orgin of the phrase "the real McCoy"?

The name is believed by most authorities to come from the inventor Elijah McCoy.

http://web.mit.edu/invent/www/inventorsI-Q/mccoy.html

One story goes that a man named McCoy invented a system for oiling railroad wheels that worked pretty well. Others copied his design, but ad-men sold the virtues of the original design saying it 'was the Real McCoy'. It was steam engine lubrication, not necessarily railroad equipment.

https://www.biography.com/people/elijah-mccoy-9391300

I googled this from Quinon.com:

"There are at least half a dozen theories about which of the myriad McCoys of America at the end of the nineteenth century is the genuinely real McCoy. Was it, as Alistair Cooke argued, a famous cattle baron? Or was it perhaps Elijah McCoy, who invented a machine to lubricate the moving parts of a railway locomotive? The broad consensus seems to be that it was Kid McCoy, the former welterweight boxing champion of the 1890s. He had so many imitators, taking his name in boxing booths in small towns throughout the country, that it seems he had eventually to bill himself as Kid “The Real” McCoy, and the phrase stuck. Now let me enter a caveat: The Oxford English Dictionary records this from a letter written by the author Robert Louis Stevenson in 1883: “He’s the real Mackay”. It’s not only in a different spelling, but a decade before Kid McCoy became famous, and almost certainly refers to the famous Scottish firm of whisky makers. So the debate must continue."

There are many conflicting possibilities:

"The real McCoy" was the inventor Elijah McCoy,born in Canada in 1844. He had many different inventions including an ironing board and a lawn sprinkler. Other companies copied his devices, but these never worked as well as Elijah's so people would say, "I want a , and make sure it's a real McCoy."
Richard Thompson, Allerod Denmark

Brewer's Millenium Edition An expression used in the USA, but formerly in Britain it was the Real MacKay. Various stories about an American boxer have been suggested as the origin of the phrase, but Eric Partridge in "From Sanskrit to Brazil" [1952] , says with probable truth that it dates from the 1880s and originated in Scotland , where it was applied to whisky, men and things of the highest quality. The whisky was exported to both the USA and Canada , where people of Scottish origin drank the whisky and kept the phrase alive. In the 1890s however, there is no doubt that it was applied to a famous boxer, Ked 'the real'
Bran Robison, Brentwood, Essex

An alternative suggestion is that it originates from the phrase 'the real Mackay', an advertising slogan used by G. Mackay and Co, whisky distillers in Edinburgh in 1870. The form McCoy is apparently of US origin.
Justine, Reading England

It was Elijah McCoy's 1st invention, a lubricator for steam engines, which made people ask for the real McCoy (at least in America). also see: princeton.edu/~mcbrown/display/mccoy.html [no longer exists]
Jason, Fairfax USA

McCoy was a Canadian whiskey smuggler in Prohibition. The Real McCoy was good Canadian whiskey from his operations, not home distilled stuff. Probably.
Stuart, London UK

http://www.guardian.co.uk/notesandqueries/query/0,,-18558,00.html