An adapter for mounting electrical outlet faceplates designed for mounting on rectangular junction boxes in hollow walls onto solid walls with cylindrical cavities and sleeves. The corrosion-inhibited fire retardant compositions are comprised of at least one fire retardant component, a suspending agent, and a corrosion inhibiting system. When the lock core is locked, the displaceable button is synchronously restricted from displacing to keep locking the lock book. The latch portion 23 is connected to the other end of the arm 22, and is latchable onto a predetermined portion of the insertion slot.