Northof54--this is an awesome question, and I cannot help set you straight! But I can speculate a bit.
I've always thought it was interesting that he signed these barrels with the English, not the German, version of the name of the settlement. It was "Christiansbrunn" in German and "Christian's Spring" (not "Christian Springs"--singular spring, named after Christian Renatus Zinzendorf) in English.
But you're right: Oerter, as a German-speaker, would surely use "Christiansbrunn." The English version was definitely not the most common usage in Oerter's time. One finds it in Moravian documents from time to time but it is unusual.
So ... what to make of all this? I think it suggests that the rifles that he began to sign and date in the mid-1770s were meant for a non-Moravian customers, perhaps even a large order (hence the numbering on some of them, which I do not believe are production numbers dating back to his start at Christiansbrunn), and it makes me even more convinced that he signed them not out of "pride in craftsmanship" as some people have written but because he was taking responsibility for them if there should be a problem with them. This became required in some later government contracts: the contracts required makers to put their names on the barrels, so if there were any flaws/problems the barrels could be traced back to the particular maker.
I suspect that the tendency to sign rifles as Oerter did (earlier than most) has to do with taking responsibility for the product (like a current label in a shirt) more than "pride in craftsmanship." And if he was providing these rifles to non-Moravians (hence "Christian's Spring" rather than "Christiansbrunn" on the barrel), it is even more likely that he would need to mark them so the product could be traced back to him.
The above is largely speculation!