Green’s Dictionary of Slang

ball of lead n.

also ball o’ lead

[rhy. sl.]

the head.

1921		N&Q 12 Ser. IX 345: Ball o’ Lead. Head.
1925		(con. WWI) Fraser & Gibbons Soldier and Sailor Words 14: Ball Of Lead: Head, (Rhyming slang).
2002		B. Kirkpatrick Wicked Cockney Rhy. Sl.