From the placard:
Small metal plugs were used by the Enola Gay weapons officer to arm the atomic bomb dropped over Hiroshima on August 6, 1945. He removed three green plugs that kept the firing circuitry inactive and replaced them with red plugs that closed the circuits. These plugs were found in the navigator’s compartment of the Enola Gay during restoration. It is not known whether the green “safe” plug is from the “Little Boy” atomic bomb or was used on a practice mission. The red plug was probably a spare.