I also found a letter from the State of Washington Office of Financial Management from 1984 with population numbers attached. According to this, unincorporated Spokane County had 349,000 residents that year, with 171,500 within the City of Spokane.
And my favorite item? A flyer from 2003 that advertised a happy hour event offering citizens a chance to hear about SRTC's Southside Transportation Plan while having alcoholic beverages. Here's the agenda that was included on the flyer:
I'm not joking, it really said that. I miss the old days.