The first country to issue a submarine stamp had no submarines in service, although it did have one boat on order from the Italian Naval Shipyard at Fiume (now Rijeka, Croatia). On Oct. 14, 1936, Romania issued a stamp on the occasion of its first naval exhibition (Prima Expozitie Marinareasca) showing a submarine identified as Delfinul (Dolphin) operating in rough seas.

http://www.public.navy.mil/subfor/u...bStamps.html

The cancel with date 25 OCT 36 , was used during the "First Naval Exibition" ( Prima Expozitie Marinareasca) of Romania , held in Bucuresti