There is an inventory of naval stores from 330 BCE, Athens
https://inscriptions.packhum.org/text/3849?hs=11015-11029

Line 321, among the various armaments in the arsenals, it mentions

"Of lead bullets (molubdides) and lead in the wicker chest, weight 6 talents and 3 mnas"

That's 156 kg in store, so enough to cast maybe 5200 bullets. Nothing to sneeze at, though probably short of the quantities you'd need to equip a large contingent of slingers.

Yes you're right, it could be for a catapult on a ship. Also ancient ships could use lead as anti-ramming devices (weights on yardarms to drop on enemy ships).

Thanks Tombelaine !