The date may say 2015, but this sushi stamp - and it's mate, tempura - were issued on October 30th, 2014 as a temporary Special Stamp for International Airmail Postcards due to a 'consumption tax rise'.
The key ingredient of sushi is a vinegared rice with a variety of fillings, toppings and condiments (usually seafood, vegetables, and occasionally tropical fruits). Sushi is traditionally eaten with your fingers, and is turned over to dip into the soy sauce so only the topping is flavoured.
For the letter S — sushi, special stamp