Jack, what makes you say it comes from the US? The sale was on UK ebay and the item was in Staplehurst, which is in Kent. In the UK, 'dulcimer' usually meant the hammered variety, at least until the Appalachian version entered the folk revival.

This one doesn't look much like an American dulcimer, although it's clearly a member of the same family, which includes the epinette de vosges from France, the Hungarian citera and the Swedish Hummel. This seems to most closely resemble a hummel, but I'm no expert.

I'm not sure about the "d.d", but on the assumption that the "Coll" in "J Marshall Coll" means a collection, my guess is she was the donor.