"Among the necessities or near-necessities were P-38 can openers"
P-38 Can Opener
Public DomainP-38 Can Opener - Credit: Jrash

Developed in the 1940s, the P38 was known as the John Wayne for its toughness. It was named the P38 for its length of 38mm, and also for the P38 Mustang fighter plane.  Some people speculate the C-ration can required 38 punctures around its circumference to open it.