Doing a search for that frequency within the state of Washington beings up 14 returns. 13 being "Washington State of" (all WSP detachments primarily on the West Side of the State) and 1 being "Cowlitz County of." I've had it programmed for ever and have never heard any traffic on it in my neck of the woods. I also can confirm that at least a couple fire depts, including mine, in the county are using 156.135. One dept uses it as a tac frequency to talk to an incoming ambulance.
WPZJ608 is licensed in Okanogan County to the
State of Washington.
I only got nine licenses on a statewide search of the FCC Genmen, five are in Okanogan County and licensed to the State of Washington.
While I am sure you have already googled it,
scannerstuff.com, reports 159.0450 to be an input for 155.5050 in Skagit/Island Counties.
Considering geographic isolation, I would say you are onto something and this is nothing more (now) than an input reused by WSP.
Again, I have had it in my machines since relocating to the Greater Spokane Metroplex a couple years back and have never heard a peep from it. I have also been keeping notes and have poured over them with negative results.
Perhaps this was something else at one time (like the 'old' air channel) but now it appears to me to be an input. Good work, thanks for pointing this out.