During WWII ships would often be presented with their own Jolly Roger flag, to which were added symbols for achievements. This example belonged to N93, also known as H.M.S. Unbeaten. Unbeaten was a Royal Navy submarine, entering service on November 10th, 1940.
The symbols on the flag show the sub ran 4 secret missions (daggers), which often included picking up and dropping off operatives in enemy territory. The three deck gun symbols show the Unbeaten had used its deck guns in the sinking of three ships. The crossed U symbols show the Unbeaten had sunk two enemy U-Boats, while the 7 stripes stand for 5 merchant ships (white) and two warships (red) sunk.
Sadly, on November 11th, 1942 the Unbeaten was accidentally sunk by RAF aircraft while in the Bay of Biscay, with all hands.
If you're looking for a unique "pirate" flag with a lot of history this is the one!