How to add a new TAB instrument to instruments.xml?

Am trying to add an instrument to the instruments.xml file. NOTE: I have confirmed Musescore is reading my edited .xml file. The issue is that I cannot work out how to get it to add a Tablature staff. The instrument shows up in the instruments list, but no matter what I do in the .xml file Musescore always creates a Standard staff.

What I did was copy the 4-string Ukulele tablature .xml entry and revise what seems like the sorts of tags needed revision for a new instrument. (NOTE: I did confirm that the Ukulele instrument does add a correct TAB staff in Musescore before doing this, and even after adding my new instrument entry.) Since there obviously insn't any "staffTypePreset" for the Mountain Dulcimer, I left that as staffTypePreset="tabUkulele" for test purposes. But, still, it doesn't work.

After many edit variations it seems that the Instrument "id" tag is significant. If I copy the Ukulele text and change just one character in the id tag then it fails to work. Which implies to me that the id tag is pointing to something in the Musescore code?? But then if that's true that implies you cannot add a new instrument by adding to the .xml by itself.

Below is the added entry - any help appreciated! fwiw; I would love to be self-sufficient in doing this, but have not found sufficiently complete documentation for the .xml file tags. So I'd take the answer or a pointer to where I could work out the answer.

I see this editor tried to read the xml tags as html, so I snipped out my instrument entry into a file and attached it.

        Strings - Plucked

              Mtn Dulcimer 3-string (Tablature)
              mtn-dulcimer
              Mtn Dulcimer (3-str. Chromatic TAB)
              pluck.dulcimer
              tablature

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