India issued a number of postage stamps of various denominations overprinted with the letters "I.E.F.". This stood for "Indian Expeditionary Force" and could be used by Indian soldiers who had proceeded overseas on active service with the various expeditionary forces from September 1914 onwards. The expeditionary forces were: I.E.F. A : 'France and Flanders' I.E.F. B: etc.
Apart from these stamps, items of military postal stationary such as honour envelopes printed in various Indian languages (wherein the sender signed a declaration confirming that the contents of the letter did not reveal sensitive information), field service post cards are avidly collected particularly when tied by a Field Post Office (F.P.O.) cancellation and/or censor stamps or labels.
Some of the rarer F.P.O. cancellations are greatly sought after and prized by collectors. For greater information see Volume 2 of History of the Indian Army Postal Service by Edward Wilfrid Baxby Proud and also the online e book - The Post Office of India in the Great War (https://archive.org/details/cu31924012679548). P.O.W. envelopes sent from different prisoner of war camps form another interesting theme.