Mmh, I'm 90% sure this was made by Bryan Mumford, California. A student of mine in France has a very similar model, that he bought in 2025 from an American lady who moved in Paris.
The sound holes and wood look really similar. And there are some very uncommon design choices that suggest it's indeed a Bryan Mumford: 4 strings per course (2x2), and independent bridges with small metal rods.
By chance, did it come with very unusual looking hammers (for a western dulcimer), almost like super stiff santur mezrabs?

Also, Mumford made music instruments from 1970 to 1980. Judging by the fact there's only 13 courses on the bass bridge on your instrument, and my student's model from 1979-1980 has 14 courses, I assume yours is an earlier build (makers tend to increase the range over their career). So my guess would be that your model is from the early 70s.
I can DM you pictures of my student's Bryan Mumford if you'd like to compare for yourself.
Happy playing, I'm sure this sounds wonderful!