Introduced in 1918. This is one of four extremely rare World War I back designs that were part of the War Series. They were only issued for one year and their rarity suggests that they were never actually released to the public, perhaps because the war ended that same year. Only a handful of complete decks are known to exist today. The other three designs are Big Gun, Dreadnaught, and Flying Ace.

Pictured below is an original box from the "Invincible" deck. But this raises an interesting question for us all: why did Mrs. Robinson name this deck "Invincible" in her scrupulously researched pamphlet when what may be the only known original box is clearly marked "Conqueror?" The final image does nothing to put this issue to rest. This is a page from an extensive sample book of all USPCC decks produced around 1917 and it offers a very nice overview of the War Series. Each deck is named on its respective sample card with the exception, of course, of Invincible/Conqueror.

  31. Handlebar                                         33. Lantern No. 1