Hallmark on Great great Aunt's gold bead necklace -mid - late 1800's

This necklace belonged to my Great great Aunt Julia; she was born in the mid 1800's, was from New York and traveled to Colorado escorting my 15 year old Grandfather in 1885-

The gold bead necklace (61 beads and 14 inches long ) is on a gold chain, quite sturdy, very fine (they are tiny) linked and four-sided - close-up is of the mark on the small round stud integral to the push-clasp. No karat marks. Any info appreciated

Note: in the close-up the beads look grainy, they aren't, very smooth. - think that effect was the flash or the resolution -