The sign accompanying the hold-down bolt. It reads
Shuttle Solid Rocket Booster Hold-Down Bolt
Eight hold-down bolts are used on every mission to keep the shuttle stack
stable and in an upright position on the launch platform. Each solid rocket
booster has four bolts attached to its base and the launch platform. Attached
to the top of each bolt is a frangible nut. The nut has two explosive charges
installed, which detonate upon command from the shuttle's computers.