Hi, I have a couple of modern (1980s) commemorative glasses each of which has an etched or engraved mark on the bottom of the foot which reads "fo" or possibly "of" as it can be read either way. There are no other makers' marks on the glasses. Does anybody know if this is a maker's mark or significes something else? I've attached a picture which shows it as best as I can considering how small the text is.