Which innovative retail distribution strategy did the U.S. military first implement during World War II that revolutionized how soldiers accessed personal items in combat zones?

The Post Exchange (PX) system has been a crucial retail component in military history, providing goods and services to service members worldwide. These military retail operations have evolved dramatically over time, adapting to changing warfare conditions, technological advances, and the needs of military personnel. This trivia question explores an interesting aspect of military retail history that shaped how supplies reached troops in combat zones.

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The Mobile PX (Post Exchange) system using converted 6x6 trucks as traveling stores
The Catalog Order System allowing soldiers to mail-order items delivered to the frontlines
The Ration Card Exchange Program permitting soldiers to trade food rations for retail goods
The Officer Procurement Initiative where officers purchased bulk supplies for their units