?
I've always used TWOK and TSFS. And it's what I've seen most other people use. I'm not abbreviating them WOK and SFS.
Four letter abbreviations are perfectly fine, not sure why a three letter-maximum is mandatory. Any series whose title requires five, however, it's getting a bit silly at that point.
Also, "SFS" would be great at word association parties, as trying to pronounce it would get 60% of the people in the room to instantly equate it as "syphilis", which 90% of the participants would probably get later that night...