Tony Hi
The F in diamond was used in the London Foreign Section, and in this instance I suspect the envelopes were included in a ships bag from H.M.S.Triumph., a 14 Gun iron armoured ship of 6640 tons.
The writer I think is Naval Cadet Edward A Baird and his ship was in Coquimbo Chile between 3rd May to 25th June 1885.
I must also confess that I am no maritime postal history expert but I do have a copy Geoffrey Osborn's marvellous book "Navel Officers Letters" published by the Rossiter Trust. Pages 54 & 55 illustrate the mark and page 60 has an envelope from the same correspondence received in Ringwood 27 July 1885. It has a different FB dumb cancel but the same 4d rate.
The book is still available from
http://WWW.rossitertrust.com price £15 but I dont know the postage costs.
Hope this is of help
Regards
Simon Burke