Egad! Thanks for the quick come-back! Your peg-box detail showed me something about my dulcimer that I did not know until just now. Your peg-box is obviously three-piece: Although the joining is as good as it gets, the grain is a dead giveaway (not implying that it is obnoxious or anything of the sort). I looked very closely at mine again and sho'-nuff it's three-piece too, except in my example the grain pattern and colors make it almost indiscernible - I don't know if that was deliberate or luck of the draw.
I'm curious, if you would further indulge me, about the nut end of the peg-box: Does it open under the nut into the body?
Comparing our examples with Jennifer's, I see that ours have a flat land between the distal end of the peg-box cavity and the scroll-head, while Jennifer's scroll-head drops straightaway into the peg-box cavity. I don't know if that represents a design change (Jennifer's is the most recent build) or just some experimental variation that Bill indulged in. As for design appeal, I guess it depends on the eye of the beholder. Regarding a practical purpose for eliminating that land, the only thing that comes to mind would be to shorten the overall length a bit.
On the tuners, from your latest photos it looks like I found a durn good match for you. A little polish on what you have and one would be hard pressed to tell the difference, old from new (if the dimensions match, that is). If you elect to replace the broken tuner be sure to leave that extension in the small hole opposite the mounting hole. Not that you necessarily need it - do it just to keep things "as was" as possible (I think it's intended to reduce lateral stain on the peg shaft). As for any adverse affect on collector value, if you keep the broken key, the instrument can be returned to where it was when it became yours. I have this ongoing project dulcimer (a Carstanjen courting dulcimer) that was unplayable as found and after crafting new nuts and replacing the fixed bridges to floating bridges now sounds pretty cool except for a body rattle problem caused, I'm sure, by unbound braces. Anyway, so far, everything I've done can be undone should some future owner want it that way - but from there it would become a wall-hanger only and/or an audio torture device. I'll be talking about this project soon in a dedicated post cuz I do need some advice on where to go from from where I'm at.
Moving on, as a three-course four-string with melody doublet, you could convert by cutting new string notches in the center of the bridge and the nut, but personally I'd hate to see you do that and rather have you opt to leave it as a four course, with or without the doublet. Of course, with the set-up you now have you can do a D-A-dd three-course with a somewhat awkward string spacing yet optionally swap to D-A-d-d four-course by simply loosening the inner string of the doublet, moving it over to the high drone slot, then re-tuning - no string removal required.
From your photos I see you may already be afflicted with the dreaded DAD syndrome. There's something to be said for someone who already owns a three-course trying a four-course. A four course is somewhat harder to master (and at first can be confusing), but it does have a different voice and to me anyway variety is a fun plus, if not the spice of life.
Well, I see I've droned on much longer than I intended to. So I'll close now with a thanks for the follow request - I'm flattered.
updated by @jim-hedman: 04/06/18 03:23:03AM