I have an Old Kraftsman dread from the 1950s. Its a Kay made cousin to your Harmony. Mine has 2 stamps inside as well. No one I know of has been able to find or figure out what them mean.
-- Joe --
Guitars:
Gibson: '08 Longhorn DC HS
Martin: '09 OM-1
Ovation: '07 Elite-T 1868T; '95 Celebrity Deluxe CC257; '95 Balladeer 1751 12-string
Fender: '91 Stratocaster Plus; '69 Mustang Competition; '08 Squier Telecaster CV50
Other: '77 Bradley Les Paul Custom; '03 Michael Kelly Patriot Q
Bass: '08 Dean EABC
Effects:
MXR Dyna Comp; Dunlop Crybaby; DOD TR3R; Ibanez MT10; Rockman Soloist
Amps:
Genz Benz Shenandoah Jr.; Peavey Studio Pro 110; Tascam MP-GT1