Mea culpa! It just dawned on me that I gave bad info in my previous reply. Maybelle Carter didn’t play the dulcimer; she played the Autoharp. In fact, she complained on the album that she had restrung her autoharp herself. And like a lot of “ideas” we get, that one turned into a nightmare, because of SO MANY strings on the instrument! She said she’d never tackle a job like THAT again.
An autoharp is a whole lot of trouble, my mother has one but I never really got into it.
I was wondering when you mentioned a hammer dulcimer and thought my memory was going or something. That is not really an old-timey instrument - at least not in the SE mountains where you would see plucked dulcimers w/ 3 or 4 strings. More of an upper-tier-state instrument i.e. New York, Pennsylvania and points west -- mostly where the Germans and Eastern Europeans were). It didn't catch on in the folk-music world here until relatively recently.