post office stone, noun phrase
In historical contexts. Any of several large inscribed stones under which early mariners calling at the Cape placed letters for collection by the crews of other passing ships. Also attributive.
[1893 H.B. Sidwell Story of S. Afr. 12The captains hit upon the idea of leaving their letters stowed safely away beneath heavy stones, engraved with the vessel’s name..till the next ship that anchored in the bay took the mails away from this queer post-office to Europe or to India.]
1973 S. Afr. Panorama Aug. 48A post office stone, built into the walls of the Castle, Cape Town, records the visits in 1607 and 1609 of the British ship, Dragon.
Any of several large inscribed stones under which early mariners calling at the Cape placed letters for collection by the crews of other passing ships. Also attributive.

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