Which technological innovation revolutionized military retail logistics during the Vietnam War, allowing for more rapid deployment of PX (Post Exchange) facilities to forward bases?

The military exchange system (PX/BX) has been a crucial aspect of soldier morale and readiness throughout history. These retail operations have evolved from simple commissaries to sophisticated global retail networks serving military personnel worldwide. This trivia question explores how military retail logistics changed during a pivotal period in military history, affecting how soldiers accessed essential and comfort items during deployment.

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Containerized PX units (CONEX boxes) that could be quickly shipped and set up
Helicopter-based mobile retail units called 'Flying PX'
Digital ordering catalogs with weekly air-dropped deliveries
Submarine-based supply chains for coastal operations