A rivet attached at the base of the button fly for reinforcement purposes. The story goes that Levi Strauss & Co. removed the crotch rivets from their jeans in 1942 after numerous complaints from cowboys about these rivets heating up considerably in front of campfires. Others were forced to remove them from their jeans to save metal for the war effort.
Source: Nouvelle de Nîmes Nº 5: The Denim Dictionary
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