1898: Signed, Halo Cumtux, Lillooet, BC
Looks like two Chinook writers at work here…
Here’s still another BC Chinook writer, from Vancouver Island, 1901 (image credit: UVic Libraries)
And reference to a likely third.
I think the headline was supplied by the newspaper editor. It uses a different spelling (kopa) from the Chinuk Wawa used by the letter-writer (copa).
The latter person writes some of the words really uniquely, and I’m sticking with my claim that this usually shows us someone writing as they really speak the language.
One of the frontier and post-frontier era customary uses of the Jargon was in pseudonyms, that is pen names when you wrote your opinions to a newspaper editor.
Another typical literary, or at least written, use of Chinook Jargon was related to this — you wrote in CJ when you were expressing controversial thoughts.
Surely this is connected with everyone’s experience that this is an informal “street” language. For Settlers who were writing (and therefore addressing themselves only to other Settlers), talking Jargon was equivalent to talkin’ “down home”. I see proof of this claim in some of the English-language grammar here; for example “he dont cumtux” sounds typical of USA English dialects, and of course in British Columbia’s Cariboo gold rush period, which led to the establishment of the town of Lillooet), Americans predominated.
Check out how Jargon and English get blended here…
Hyas Klawhawyuh Kopa Nika. 
Editor Prospector: Mr. Cumtux  thinks he cumtuxes, but he dont cumtux as much as he thinks he cumtuxes. Pe cultus copa nika spose yakha sylose. 
In the first place we can call the several white witnesses to testify to the truth of what they saw and to swear that the Indian who pulled out the bottle was drunk at the time. Who ever heard of an Indian carrying around a bottle of water in his pocket and treating several of his friends who would smack their lips at it and wish for more. Again, the whites were not watching for any unlawful action on the part of the Indian, but were sitting in the store at the time and a few saw the bottle passed around just outside the window.
We have been asked since why we did not “nab” him. We had no business to “nab” him, Like the true business man we did not want to hurt his feelings. He might have stopped “buying” you know. This is the common thing among tradesmen — and we are only human after all. We have a good many of humanities [sic] failings. We have also noticed the same failings in several government officials.
Now, if Mr. Cumtux thinks we care how others see us, or what they think of us or say of us, he is badly left. We know humanity too well to imagine that one has ever a good word to spare for another if he thinks that others is getting a little ahead of himself in the affairs of life. And finally, we would not be so conceited to sign ourself Cumtux, as if to make known that we knew everything, so we will sign ourself and let the world judge. HALO CUMTUX. 
— from the Lillooet (BC) Prospector of November 4, 1898, page 1, columns 3 & 4
Hyas Klawhawyuh Kopa Nika  = hayas-ɬax̣á(w)ya(m) kʰupa náyka = ‘(it’s) very pitiful to me’.
Cumtux  = kə́mtəks = ‘know(s); understand(s)’.
Pe cultus copa nika spose yakha sylose  = pi kʰə́ltəs kʰupa náyka spus yaka sáliks = ‘and it’s unimportant to me if he’s angry; I don’t care if he’s mad’. The spelling < sylose > (elsewhere a now-antique variant of “xylose” in chemistry) is very unusual in its phonetics. The first vowel, < y >, is easy to explain as an instance of American Settlers’ known tendency to perceive Chinuk Wawa /á/ as dialect-English /áy/. In fact this same Jargon word was often written as < silex > and so forth. But the final < ose > suggests not so much the CW final /iks/ as a pronunciation [os]. And that happens to correspond pretty exactly with the Salish suffix -us ‘face’; now it just happens that local St’át’imcets (Lillooet Salish) uses that suffix in an apparently common stem qlíl-us- ‘(have/make) an angry face; angry looking’! We’ve often seen testimony, and direct evidence, that Chinuk Wawa usage varied from locale to locale, and that a given area’s most frequently spoken languages supplied words to it. So maybe here < sylose > is a genuine Lillooet-ism!
HALO CUMTUX  = hílu kə́mtəks = ‘doesn’t understand; doesn’t know’.
In the same era, there was a silver-mining claim in the Chesaw, Washington area with the similarly slangy name of “Don’t Cumtux“.