The model 12X light bar was offered to the public in 1970. It features many of the same design ques from the model 12 in that the outer rotators are housed under a polycarbonate lens, the motor is mounted on the drivers side. The difference in the model 12X to the model 12 is that the engineering at Federal Signal took the design of the tall 3 step cascade mirror and split it horizontally. They added a brace between the split mirrors and also had a stabilizing mount toward the inside rear corner.
The bottom mirror faces forward and the top mirror faces rearward. Many departments took these light bars and made the split mirrors both face forward. They often removed the metal brace between the mirrors. This metal brace became rare and I have re-manufactured them.
Photo's courtesy of fdny5555