Ok the reason I referenced that time frame, I ran across a radio shop that had a bunch of Motorola trunk mounts, back in 1980-81 I wanted to buy one but he said they were wide band and could not be used any more, they were the green colored enclosure and as I recall they were solid state or maybe a tube/solid state hybrid, they were only about 4-5 inches tall and no ventilation holes for the tubes, see why the time frame? So they weren't from the 50s or 60s.That change was in the late 50s-early 60s, not the 70s or 80s. Channel spacing was 30 kHz with 60 kHz separation between adjacent assigned frequencies, i.e. 154.310, 154.340, 154.370, 154.400, 154.430, etc. and adjacent assigned frequencies would have been 154.310, 154.370, 154.430, etc.
It became 15 kHz, i.e. 154.310, 154.325, 154.340, 154.355, 154.370, 154.385, 154.400, 154.415, 154.430, etc., with 30 kHz separation between adjacent assigned frequencies.
Soon it will change again to 12.5 kHz spacing and 25 kHz separation for analog, and 6.25 kHz spacing with 12.5 kHz separation where applicable for digital. Check the VA STARS system for the layout of their freqs for their vhf high band trunking. That'll give you an idea where it's all going.
Us old guys remember this kind of thing because we were still wet behind the ears when it was happening.