For BCH, and any other readers who are interested:
According to the "History" section at the Waterbury Button Company website, that company changed its name to Waterbury Companies (note that it is plural) during World War Two. Thus, the longtime "Co." abbreviation in the backmark became Co's.
I did some additional research to find the exact year, which IF I'm recalling correctly turned out to be 1943. Unfortunately I don't remember where I found that info, so to be sure about the 1943 date you'll need to doublecheck my foggy memory of it. I do feel certain that if you went with 1944 you'd be within one year of the correct date of the name-change to "Companies" and the emergence of the Co's backmark.
Edit: Here are a photos showing two variations of the Waterbury "Companies" backmark. One has "Companies" spelled out, and the other has the abbreviation Co's. Note the "INC" for Incorporated. Also, I've seen a third variation, which says simply Waterbury Co's Conn" (without the "Inc.").