This "Milton" letter was sold for £400 at the 13 December 1967 Robson Lowe auction. I don't know who owns it now.
Here is the description in the 1967 catalogue: 1865 (28 July) cover with G.B. 4d. in combination with N.S.W. 6d.
both cancelled "B56". The enclosure is written from "H.M.S. Curaçoa, Ovalau, Fiji Islands and is
endorsed "Posted per H.M.S. 'Esk' at Aneityum, New Hebrides Islands". A photostat of the enclosure is included
with the lot. A great rarity. Photo in color on front cover. Estimate £300.
Information thanks to Alain Millet. And here is the
original page of the auction (klick on the image which shows for a blow up) - thanks to "JGI"
H.M.S. Curacoa left Sydney on June 4th, 1865 and returned on October 13th, 1865.
She was met by H.M.S. Falcon with English despatches for Commodore Wiseman at Tongataboo on July 18th.
Curacoa reached Aneityum on 6 August and anchored in the bay between southern Aneityum and Whale Island, now Inyeug (with the air strip)
which is now sometimes called Mystery Island. H.M.S. Esk was already there and the mission brig 'Dayspring'
with the Reverends Gordon and Patton. (Source: "Jottings during the Cruise of H.M.S. CURAÇOA among the South Sea Islands in 1865"
by Julius L. Brenchley, M.A., F.R.G.S., London, 1873).
"H.M.S. Esk, which sailend hence on the 18th instant (July 1865), has gone down to the Fijis, with the English mail,
for the purpose of meeting the Curaçoa, Commodore Sir W. Wiseman."
(Sydney Morning Herald, Friday, 21 July 1865)
Esk returned to Sydney on September 21st, 1865.