An exemplary fitting for a mullion includes a first bracket, a second bracket nested with the first bracket, and a lock cylinder assembly selectively coupling the first bracket and the second bracket. The nested brackets include a set of interleaved walls, and openings in the walls cooperate to define a pocket in which the lock cylinder assembly is seated. The lock cylinder assembly includes an armature having a first position and second position. When in the first position, the armature prevents removal of the lock cylinder assembly from the pocket such that the lock cylinder assembly prevents separation of the brackets. With the armature in the second position, the lock cylinder assembly can be removed from the pocket, after which the brackets can be separated from one another.