I know a few people who have bought New Harmony dulcimers over the past year or two. They rave about the big, full sound. I haven't played one myself, but hope to soon. I met Rick (the luthier, not the business/PR guy) in Zoom when New Harmony was a vendor at the virtual Berkeley Dulcimer Gathering. He seemed to know most of the other dulcimer makers in the country and was familiar with the variations in their designs. I'm not ready to buy another dulcimer right now, but we discussed my personal preferences, and he was certainly willing to build a dulcimer to please me.
Several luthiers offer a three-string option, and a few (like Modern Mountain Dulcimers) only make dulcimers with three strings. That approach may be in the minority, but it is not unheard of.
As @gail-webber explains, the accutune bridge is a way of adjusting the compensation for each string individually, providing greater accuracy in intonation depending on tuning and string gauge.
Perhaps just as importantly, the New Harmony design does not really use an end block. Instead the strings latch onto an undercut bridge with several inches to spare before the end of the top. That design allows the sound board to resonate a lot more than it does on a standard dulcimer. The resulting sound resembles not the traditional, high silvery sound of older dulcimers but the warmer, round sound of a guitar.
As for the hole in the head, I assumed it was simply so that the instrument would be easy to hang on a wall on a simple peg, like those traditional Shaker chairs.
Dusty T., Northern California
As a musician, you have to keep one foot back in the past and one foot forward into the future.
-- Dizzy Gillespie