According to info I saved from another thread, which was attributed to Ken (bajaespuma), GE put the letter code in the model number between 1961 and 1968. So, for your fridge, Y is 1964 and D would be February. Like you say O was never used for either month or year designation. To confuse things more the GE chart, from GE's website, I have shows a two letter code to designate year only between 1961 and 1968