During the period this guitar was made, high amp gains and even high amp volumes had not reached the masses of the consumer guitar market; So 60 cycle hum from ungrounded strings was not a big concern among most manufacturers. Ironically, the manufacturers thought that if a person wanted more volume he / she would simply buy a larger amp that would provide the clean / clear headroom that was industry standard at that time and didn't accentuate 60 cycle hum from ungrounded strings. But that was soon to change as more and more Rock and Blues Players were discovering the tones of high amp gain and high amp volume, where 60 cycle hum from ungrounded strings become a noise problem.
It's a piece of cake putting a strings ground on a Harmony guitar with these DeArmond Golden Tone pickups. There is loose clearance under the pickups base plate that a wire can be slipped under and connected to the strings, ...even if simply sitting in one string slot of a wood bridge.
I use a length of guitar string for the job because I can choose a string gauge that will slide under there yet still fit tight enough when coiled up. The coiling will not be evenly flat so any number of places on the coil will come in contact with the pickups baseplate for a good ground: