In 1941, Roberts Dairy of Nebraska released a set of twelve milk bottle cap covers, each containing a different national defense pledge. The two-color cardboard discs, referred to as "stamps," measure approximately 1.53 inches in diameter. The image is based on the cover of Superman #6 (September–October 1940), penciled by Joe Shuster and inked by Paul Cassidy. The Roberts Dairy "stamps" are the earliest known examples of Superman milk caps. Milk cap collecting peaked during the 1990s with the commercialization of a Hawaiian game commonly known as POGs.

Each cap states, "Save All Twelve Stamps - Become Member Of Roberts Superman Defense Club". The back of each cap reads, "SAVE THIS STAMP. When you have collected the complete set of 8 different stamps, you will be enrolled as a member of the Superman Defense Club of America." A complete set of twelve caps could be redeemed for a premium brass badge.



For ten cents and two milk caps, members could receive a premium Superman Secret Chamber Initial Ring made by the Ostby & Barton Company. Each adjustable brass ring contains a secret compartment that reveals a Superman, Inc. logo sticker. Club members could select a single letter initial. Less than twenty of the Secret Chamber rings are estimated to exist.

On May 20, 1941, President Franklin D. Roosevelt signed Executive Order 8757 Establishing the Office of Civilian Defense (OCD). Regional programs were organized throughout the United States in cooperation with OCD and the Department of War. Supermen of America advertisements began encouraging readers to buy Savings Bonds and Defense Stamps in Action Comics #43, on sale October 14, 1941.

The Superman Junior Defense League of America was marketed by various bread bakeries and radio stations that aired The Adventures of Superman. Local sponsors of the Superman Victory Kid Club included Boskman Variety, Village Pharmacy and Fountain, and Wigmore Dairy.

Roberts later produced a series of twenty "Code Note" milk caps for the Supermen of America Club. "Code Note" caps consist of two layers with a stapled die cut pull-tab, know as the "Perfection Pull Cap." Each Roberts Milk "Code Note" contains a different coded message. "Superman's Secret Message!" first appeared in Action Comics #15 (August 1939). Coded messages were intermittently printed in issues of Action Comics and Superman.