Actually, McCoy seems to have come to country-tuned instruments rather gradually. Before that, he would use, say, an A-harp in E, and an E-harp where he needed a D# or a B major chord. A fan heard this and told him "I know how you do that - you tune Draw 5 up a semitone!" That fan didn't know he'd just invented country tuning; Charlie took the idea and put it to good use.
The "Wizard Winslow" is correct.When Charlie was doing Londonderry Aire (Danny Boy) with 2 harps. I was using 1. There's no mystery to this (so-called) country tuning. I discovered the tuning in 1959 and I'm sure I wasn't the first. btw, I also tune UP the 9th reed. So, in retrospect, getting to the bottom of ANY legend can sometimes have surprising (and sometimes disappointing) results.