Interesting views we have of the PNW. I'm at the north end of the "distribution" and the Russians had cattle up here early on, along with straight iron knives. If I recall correctly Shelikov arrived on Kodiak in 1792 and the first cattle arrived around 1810 or so. We still have a remnant feral herd of "Russian" cattle on Chirikof Island that has caused quite a stir among heritage cattle enthusiasts for their "pure" bloodline. One ranching operation managed to collect some and transport them to Kodiak with intentions of breeding into their own herds for genetic advantages. I helped work a herd of them a couple of years later, and the bulls in particular still had a lot of "feral" in their outlook on life.
I've occasionally been charged by bulls in past years, but the event was always short lived. These things prosecute the charge for a considerable distance, and repeatedly. Usually at the first sight of you. Egad.
I digress. Again.
I'm relatively certain that the Russians also transported cattle to Sitka in southeast Alaska and Fort Ross in California pre-1850, and US ships regularly coursed up and down the Pacific coast before 1850, with beef hides from California being the principal commerce for delivery to Chile or around the horn to New England.
And with the Hudson Bay company and other trading at Astoria and points north pre-1850, it's easy to assume that if they were trading guns they were also trading horns in that era, even if the Natives didn't make the horns themselves.
Shake it all together and spread in a pan, and I think the mix could easily lead to access to horns pre-1850. In fact, I would call it almost certain. And with their penchant for art and decoration, I can't see them leaving a plain horn plain.
This one is certainly unusual and well done, but I can't convince myself it's outside the realm of possibility. It would certainly be good to check with the various museums for other examples of horns for comparison.