Herodia rose gold watch: what does caseback marking mean?

I own a vintage Herodia handwound watch with a rosegold(plated?) case. There are no hallmarks visible on the case, the caseback or the inside of the caseback where you can usually find details on the quality/type of gold used. There is however a 90 in a square stamped on the caseback (90, not 9C). I've seen this on other gold Herodia watch casebacks on the internet but I can't find out what that 90 means. Can somebody enlighten me? Thanks in advance.