I don't know when rings Began to be stamped, but I'd be willing to bet that it was before 1906. From what I can tell, after that they were not required to be stamped, but if they were stamped, they had to be within a certain margin of error of the fineness stated. (This law was amended in the 1970's and 80's, and the fineness tolerances were made more stringent.
I have found Brass rings that were stamped "18" and "18K" before--even though they weren't...and I feel that they were likely pre-1900 rings, since no other items newer than 1880ish were found at either site.
So I'm just going to play Devil's Advocate here, and say that the ring was made around the time the date suggests
Regards,
Buckleboy