Well, quite a bit has changed since my first few posts in this thread, 4 1/2 years ago, then I had four RPOs in my Redwood Empire Two county postmark collection. And I can answer my own question.http://goscf.com/t/14858&whichpage=4#379246
The train line reached Willits in 1902, my earliest Willits & San Fran is Oct 27 1903. I also have a Ukiah & San Fran struck on Jun 15 1903. Ukiah was the next station south of Willits, 22 miles.
Now there are 52 cards and covers all with North West Pacific RPO postmarks, of which most have been posted.
I snagged these four cards during Jim Forte's clearance sale. Three of them are all addressed to the same person, and posted while aboard the train. The author kindly noted the town nearest when he mailed the card, about 15 miles apart.
I guess Jim must have been quite busy when he identified these postmarks using the RPO catalog numbers. Here are the Wilits & San Fran RPO catalog listings.
985-U-1 is 30 mm, 985-U-2 and 985-U-3 are 29mm.
These two cards were identified as NWP 985-U-1.
The third card was identified as NWP 985-U-2.
This is the fourth card posted 13 months later, it was identified as NWP-U-3. Personally, they all look the same to me