[Tubesforme]

I know that prior to 1930, Philco branded tubes were manufactured by RCA. Then Philco changed over to Sylvania manufactured tubes, branded Philco. I know all of the EIA codes and meanings of date codes after about 1940. However I am curious about the 1930's Philco branded tubes manufactured by Sylvania. I have many different tubes of this type, all four or five pin types. The codes are like these examples: K1, F0 and OB. I have never come across any info on these two digit codes. Can anyone tell me the years this type of code stands for as well as maybe the month of manufacture? Nothing to loose sleep over, just interested, that's all. Thanks guys.