This cover offers several points of interest.
First, it was sent from the office of the CO, Marine Detachment, Port of Spain, Trinidad. Second, it was addressed to his opposite number in Kingston Jamaica. Third, it bears the receiving stamp of the Frederick Snare Corp, who were the main constructors of the US base in Jamaica.
Finally (and Nick I knew I would come across it one day), it bears the franking 30 Jul 1941 of USS Osmond Ingram. For avid readers of the forum over the last couple of months, you will recollect Nick posting something about this ship (but I couldn't find the original post). She was an old destroyer, launched in 1919, which was brought back into commission in 1940 and converted to a seaplane tender. At the time of this cover, she was acting in that capacity in the area of Trinidad, Antigua and Puerto Rico. Later she was reconverted as an escort destroyer and finally as a high speed transport for amphibious landings in the Mediterranean. She continued to serve in the Pacific and was decommissioned and scrapped in 1946