Green’s Dictionary of Slang

1. a bunch of seals (hanging from a watch chain).

1859		‘Hundred Stretches Hence’ Matsell Vocabulum 124: The thimbles, slangs, and danglers filched, / A hundred stretches hence?
1895		Crowe Aus. Sl. Dict. 11: Danglers, a bunch of seals.
1901		Argus (Melbourne) 20 Sept. 6/4: The red ’un is a gold watch, the white ’un only silver, while a gold chain is a red jerry, and the seals danglers or onions.

2. the testicles.

1934		‘J.M. Hall’ Anecdota Americana II 97: D stands for Danglers / More coarsely termed nuts.
1995		A. Higgins Donkey’s Years 122: His pyjama trousers slipped down below his knees. The rear exposure was sensational [...] the hairy crevice and billygoat’s matted danglers and dingleberries.