Walter Miller - Earliest 4 string Equidistant Chord/melody Player?

I have just received the DVD 'The West Virginia Hills - A Tribute to the Dulcimer'. One of the players featured in the film in Walter Miller who plays 4 string equidistant across the fretboard. He has been playing this style since the early 1930s and builds his own dulcimers (some 6 string equidistant). The tuning that Walter uses in the film is G,d,d,d (D,A,A,A but 4 steps up). He plays a number of tunes including the track behind title credits. Here is Walter when the film was made playing one of his own 4 string equidistant dulcimers:

And here he is back in the early 30s, with what looks like a 6 string - I don't know the tuning he used for that instrument but each recording I've heard of him playing so far has been in a 1-5-5-5 style of tuning:

Here is a photo of the bridge of one of his early dulcimers:

And here is the 4 string equidistant type he was making when the film was shot (about 1994 I think)

Robin