Green’s Dictionary of Slang

plate-fleet n.

In phrases

when the plate-fleet comes in [SE Plate fleet, the fleet that brought home the annual yield of silver from the Indies to Spain]

when one finally makes a fortune, ‘when one’s ship comes in’.

[UK]B.E. Dict. Canting Crew n.p.: Plate-fleet comes in when Money comes to Hand.
[UK]New Canting Dict. [as cit. c.1698].
[UK]Bailey Universal Etym. Eng. Dict. [as cit. c.1698].
[UK]Grose Classical Dict. of the Vulgar Tongue.
[UK]Lex. Balatronicum.
[UK]Egan Grose’s Classical Dict. of the Vulgar Tongue.