Identify City On A R.p.o. Postmark

I'm trying to identify this postmark on a postcard I have.
I'm certain "SEAT" is Seattle. But "BELL PORTT" is confusing. Based on Postcard content, it originated in or near Seattle Washington. The recipient address was Portland Oregon.
But I don't think the other city in the postmark is Portland Oregon as the "BELL" makes no sense. Also, There are two "T"s in PORTT. Additionally, I found (online) a R.P.O. postmark of "SEAT & PORT". If the two starting and ending cities of that railway were Seattle and Portland, that would be the postmark used.
I'm also assuming "NORTH" is an abbreviation of the railway. Not very helpful as many, many railways had "North" in their names.
Any help would be appreciated.

MPOS catalog number W-15-c
Bellingham, Port Townsend & Seattle, 182 miles, Puget Sound steamboat
Rarity "V", which is quite good for a 20th century postmark.
Please show up the entire postmark/message side, thanks.

Also, "North" is the direction of travel, not part of the name of the railway.

That month and day is my birthday. I expect gifts from everyone next month. lol I'm kidding.

You should acknowledge the basic mis-step made at the start, causing some of the confusion. There was no "PORTT", the spacing difference between the two Ts is obvious when one is carefully looking to tell what it is, and the second T has a period (.) after it. Placement of ampersand (&) reinforces this.

So, looking for a "PORT T. " would have likely allowed you to find Port Townsend. Lesson, practicing "attention to detail" is a lifelong learning skill (speaking to myself as well).

Thank-you for the help.
I did notice the space in PORTT, which caused me to consider Port Townsend. I also considered Bellingham. But I could find no railway link between all three cities that wasn't two different railways. I didn't think about ferries.
After I posted this, I did find a reference to ferries so considered it.
But my understanding was a R.P.O postmark was the two "end" cities. But I suppose this is a triangle. Or, is the ferry between Bellingham and Port Townsend combined considered an endpoint?

My collection is one my step grandfather collected for my brother and me in the 70's and 80's. My brother died and I am looking at it seriously for the first time in years. So there is a lot I need to learn. I have started combing the two collections of covers. Many of the First Day of Issue, postcards and other mail pieces are in no protective sleeves of any kind, so I'm making that a priority.

Also, since I am new to all this, can you please explain
MPOS catalog number W-15-c
and
Rarity "V"
Thanks

Here are the screen shots I sniped from the Mobile Post Office Society catalog, a combination of two pages of the 1399 page catalog.







The number W-15-c is the number of the CDS, to differentiate between two quite similar markings as shown above.
The "V" rarity rating indicates that it is scarcer than an IV, III, II, or I. Probably due to it being a Steamboat and it's two-year life span.

Edit for spelling

Thanks. Great information.

The example you posted and the date on the OP cancel show a four year span.

A late use?