In the early days of WWII, the Army designed C rations. What they needed to go along with the canned food of C rations was a very small disposable can-opener. Designed in 30 days by the Subsistence Research Laboratory of Chicago, the tiny folding can opener was perfect. As the troops used it in field conditions, it was realized that it was the perfect invention. It was tiny, almost weightless, and it did the job. Experience proved that 38 punches of the can opener would open a C ration can, and it became the P-38. It also was perfect as an emergency screwdriver, as a cleaner for boots and a million other tasks. Link to P38 can opener patents.