There haven't been many posts lately for old K's so I thought I would chime in with a picture or two.
Here are a pair of 21" (20.75") K's we use in the PSO.
No one I've talked to at Zildjian can give me an answer about the "F" in the stamp. "G" is for Gretsch, which I've usually seen on the edge of some cymbals when Gretsch was dealing/selling/importing them.
I was told by Leon at Zildjian, "I don't know, maybe all those were made on a Friday, so they put an 'F' in the stamp". He was joking of course, but I'd like to one day find out the answer.