Adding extra frets to the traditional diatonic dulcimer fret pattern is nothing new. Most common are the 6 1/2 fret and the corresponding 13 1/2 fret an octave higher. These two frets are so common that one could say they have become standard issue on most dulcimers. These two frets are standard on the dulcimers I make unless someone requests otherwise.
I believe it was Howie Mitchell who popularized the 6 1/2 fret. Some vintage dulcimers did have a 6 1/2 or other additional frets but they were relatively uncommon. Howie added the fret while teaching himself to play the dulcimer in D-A-D tuning. With the addition of the 6 1/2 fret one can get a C sharp on the top and bottom string when tuned D-A-D. This also allows playing a G sharp on the middle string.
More recently many players are using a 1 1/2 (one and one-half) fret. In D-A-D a 1 1/2 fret will provide F natural on the top and bottom strings and C natural on the middle string.
Many years ago, before I used a 1 1/2 fret or even a 6 1/2 fret, I added what I call a 1/2 fret to some of my dulcimers. I call this fret a 1/2 fret because it is placed between the nut and the first fret.
In D-A-D this adds both D Sharp and A Sharp to the fingerboard.
I found it very easy to adjust to having the 1/2 fret on the fingerboard because it was below all the other frets.
The combination of the 1/2 fret and 1 1/2 fret adds a lot of musical possibility to the lower position on the dulcimer fingerboard.
Here is the standard D Major Scale played in D-A-D:
With the addition of the 1/2 and 1 1/2 frets a chromatic scale can be played:
By using the 1 1/2 fret it is easy to play a G Major scale or a D Mixolydian mode in the lower positions while tuned D-A-D:
And here are a few chordal possibilities:
In the first measure are two inversions of a B Major chord. The second measure shows two inversions of a G minor chord and the third measure shows two inversions of a C Major chord.
These are just some of the possibilities. I often play B flat, E flat and other chords while tuned D-A-D using the 1/2 and 1 1/2 frets.