Richie, I reckon you've already checked out Charles Wolfe's "Classic Country: Legends of Country Music", Routledge, 2001, but in case you haven't, it contains chapters on the Delmores and the Brown's Ferry Four. I don't have it to hand but I recall that Wolfe tells the story of how the Four was formed to fill a gap in a broadcast schedule created by the drafting of the station's former gospel quartet into the armed services. Or something like that. I think Wolfe might give a date, but perhaps it's his information that you're querying?