I'm gonna say this whole thing got started because of Bob's routine of breaking a string during "Hard ain't it hard" and changing it before the end of the song. This was, of course , part of the show as he did this on a regular basis. On the DVD "Everybody's Talkin" he explains, "The guitar is great...but the strings are....crappy"
Bob was an authorized Martin dealer.
This being the case, I assume that he was able to buy Martin strings at a discount.