I believe Clay county still transmits a PL 88.5 on 155.73, however the cars for the agencies they dispatch for talk back using carrier squelch.
I can't say for sure, but I'm going to guess that agencies still transmitting PL on 155.73 are leftover from the era when most small and medium sized towns and counties in Missouri used 155.73 as their primary channel.
Also keep in mind that during this same era, most mobile and hand held radios used mechanical reeds to generate a tone. Multiple tones would have required a larger housing to accommodate each additional reed. This would be a problem for handheld radios. Radios capable of multiple tones were not common.
When agencies began moving to their own 'private' channels, they most likely maintained the same PL tone on the new private channel as they used back on the '73' channel because radios were still restricted to just one reed.
In cases where only part of the '73' users were moved to a private channel, the remaining '73' users might still use equipment that expects a PL to be transmitted. An example of this might be when a sheriff's department moves it's own communications to a private channel, but the small towns it dispatches for remain on '73'.
As for 155.37 and 155.475, I don't think I know of anyone using a PL or DPL tone.
Those channels weren't meant for exclusive use.
