After looking at your pictures once again, my best guess is that this dulcimer was made by Leonard Glenn. The thick, stubby tuning pegs were pretty much a standard feature of Glenn dulcimers. The preprinted interior label was most likely glued on by Leonard when the dulcimer was being built to identify it with Frank Proffitt's name prior to sale. Ralph Lee Smith and others have documented the working relationship Proffitt had with the Glenns, and your dulcimer seems to verify that relationship with one of the surviving instruments. Proffitt's popularity in the early 1960s on the folk festival circuit would have made a dulcimer bearing his name a desirable purchase for a young folkie. It sure would be nice if we knew how many dulcimers were actually built by Frank Proffitt and how many were built for him by the Glenns.
In either case, the dulcimer is a part of dulcimer history, and it would be a fine addition to someone's dulcimer collection.
Thanks for the info! The dulcimer has been sold and will be in a museum in VA. Very happy that it will be displayed and have the love it deserves.