Hi Tim;
Thanks to Dan Goad for mentioning my article.
We don't refer to tunings from left to right, or highest to lowest, but rather from bass string to melody string.
Normally we reference with the instrument on your lap, tuning head to the left. Some older books use melody to bass string order though.
The conventional dulcimer has three courses of strings, and one of which can be doubled or even tripled -- bass, middle drone, and melody. Yours has a doubled melody string. Doubled course notes are not mentioned in tunings, unless the two strings of a course are tuned to different notes -- DdAd would be a doubled bass string, one tuned D, the other d; then a middle drone course tuned A and a melody course tuned d.
The difference between D', D and d are octaves. They are all three the same note, just an octave apart in pitch. The D is D3 of a piano; D' is an octave lower than D, and d is an octave higher than D. The sequence goes C', D' E', F', G', A', B', C, D, E, F, G, A, B, c, d, e, f, g etc...
There are 4 course dulcimers, but the strings are spaced equally apart.
The most common tunings are
DAd - Mixolydian Mode
DAA - Ionian Mode
DAC - Aeolian Mode
DAG - Dorian Mode
Each is used for different kinds of songs -- major or minor key, etc.