I've been taking frequent drives over Cornelius Pass for about the past 6 months or so. I took another one after work today and had these observations, in addition to the ones from xonhulu:

See that bare wooden post just to the right of the school crossing ahead sign? It's positioned more or less directly across the road from the END OR-127 SOUTH shield. I don't know for sure, but it seems that this would be an appropriate place for a BEGIN OR-127 NORTH shield that has yet to be mounted. This isn't too far from the offramp from US-26, so this seems an appropriate place to stick one.

Here's a close-up the sticker on the back of the BEGIN OR-127 SOUTH assembly. Seems these signs were manufactured in July 2019 and have been sitting in storage for almost 2 years.

Keep an eye on this bridge. It was built in 1946 when Cornelius Pass Road was realigned onto its current route. It's slated to be
replaced by one that can support freight traffic, with wider shoulders. However,
the estimated completion date is December 31, 2023, so it won't be ready for prime time just yet. Until then, the bridge is still being maintained by Washington County.

Interesting story about this photo -- a few seconds after I stopped taking these photos, I saw a car pass both the car on the right and the car behind it across the double yellow lines. If I'd been traveling down this road just 30-45 seconds later, my car might've been totalled and I might've been seriously injured or worse. Too bad I wasn't quick enough to snap a photo of the guy's license plate.
You can see a BGS in the distance of that first shot. Here's a close-up of that (still on EB 30):

From the other direction (WB 30):

Since you took a trip down there, ODOT has installed signs pointing to Hillsboro beyond these BGSes. Here's the one on US-30 eastbound:

I didn't get any standalone shots westbound, but you can see it peeking through in this photo:

This is what that looks like in Oceanside:

And while I'm at it, its counterpart in Tillamook:

Huh, I've never seen Series E on an END banner before. Oregon mostly uses Series C.
OCGuy81, you're right that END signs were once basically non-existent in Oregon. But in recent years a few have been going up around the state. Besides OR 131, few others I can think of are:
US 30's and US 26's west ends
OR 104 by Fort Stevens
both ends of OR 469
OR 219 in Hillsboro
But I know there are some more in other parts of the state. They're still uncommon, though.
BEGIN signs are even rarer. Before these, the only ones I was aware of were the pair of BEGIN EAST US-20 signs in Newport. From GSV:

