Green’s Dictionary of Slang

marble n.

In phrases

roll one’s marble (v.)

(US) of a woman, to masturbate.

2002

‘xpi3’ ‘Male & Female Masturbation Terms’ at Spankmag.com 22 Oct. 🌐 Rolling the marble.

SE in slang uses

In phrases

good marble (n.) [horseracing use: a good position at the starting gate; ult. next]

(Aus.) an advantageous position.

1966

D. Niland Pairs and Loners 80: My marble’s good there for a cushy job and a few quid picking apples and pears. It’s a cop.