An Unknown Call Sign

I found this posted online for auction. It was listed as "unknown" call letters. As you might expect, that was all too enticing a mystery.  Clearly the important part is that mic, since over it is a call sign. But the photo is washed out. The scan seems to be of decent quality so I blew it up. Then I added contrast and then started playing with the color filters.

So that first letter is pretty rough, probably a "K". But then the next three letters look like "TSF" making the calls either WTSF or KTSF (assuming this is a US station.) But KTSF is a TV station that signed on in the 1970s. There also were no radio stations of note under the calls of WTSF. I am for the moment... stumped. Unless one of those blobs is a number and not a letter.. i.e. experimental calls.