Prior to 1920 mail from Pitcairn Island was erratic, depending on passing ships. In 1927 the New Zealand Government opened a postal agency on Pitcairn using New Zealand stamps.
Between 1920 and 1927, a more regular mail service was introduced; with letters in theory allowed free postage, as long as they carried a cachet indicating their origin.
This rare cover to England carried a 'POSTED IN PITCAIRN ISLAND / 1924 NO STAMPS AVAILABLE' cachet in black. On arrival in England however, the postal authorities applied two Postage Due stamps - perhaps they had not dealt with such a cover before!