Pieces hallmarked by KO appear to be extremely scarce, and KO would seem to be another "unidentified CES retailer."
The master's chop marked is difficult to determine from the photo. It appears to be (in the Japanese style of writing)
珍北. It also looks very much like
珍85. Some clue may be found in the same chop mark appearing on a piece retailed by LYH (see the CES page:
http://www.925-1000.com/chinex_marks.html). It would seem probable that this master worked for more than retailer in the same city.
If LYH is Lee Yee Hing of early 20th century Hong Kong, then KO might be a contemporary by virtue of retailing works by the same master
珍北 / 珍85.
I would call this surmising a stretch rather than a hypothesis. Perhaps it will lead someone else to provide a more insightful comment.
Regards,
David Ross