D'Addario Flex Lock Strap Blocks are a hassle-free strap locking system designed for players who don't want to modify their existing end pins. Simply slip the Flex Lock Strap Blocks over your existing strap buttons and you’re ready to go. Made of flexible, durable rubber to ensure a secure connection, Flex Lock Strap Blocks fit most manufacturer end pins on the market and are available in black or red 4-packs. Inspired by the classic Grolsch bottle cap strap lock solution.