it is an evolutionary change and a technological advance of a new item. Remember before the Model 17 all lights were stationary and either were constant or flashed. The obvious improvement was a rotating light that provided 360 degree visibility. The 17 was born. Then competition started pushing changes, and what's better than two beams spinning? 3 or 4 beams spinning, hello 184 and similar lights. How about the the lights pointing up and down (the 175), another change. The Visibar was the first light bar. the original design of the bar was to hold a sign between two rotating lights and a way to mount it all as a unit. (1963). It changed with the next design putting a speaker in between the lights (1964). Again the point of the bar was a way to mount multiple devices to the roof easily. The designer of the bar Earl Gosswieler then changed the design and enclosed it as the Twinsonic. The invent of the parabolic mirrors was based on the way the CP25 speaker reflected the light as the rotators spun on the Visibar. Seeing the wasted light, he added the mirrors and enclosed the whole thing for ease of care and to protect the mirrors. The next step was the Aerotwinsonic, which was merely an aerodynamic version of the Twinsonic. The 60 watt Par 36 bulbs necessary for a good effect off of the mirrors came with a negative, power draw. The Aerodynic moved from a chain driven setup with par 36's to smaller bulbs and motors that not only provided better light in all directions (the X mirrors were a flop) but provided a means of changing light patterns by sequencing the individual rotators. Each step was an improvement above the last. It may not make as much sense to us today with hindsight, but they were making something that had never been made before and were learning over the years.
I have to credit STANSDDS for much of that information.
