It's a radio callsign (https://en.wikipedia.org/wiki/Call_signs_in_the_United_State...)

> The FCC policy covering broadcasting stations limits them to call signs that start with a "K" or a "W", with "K" call signs generally reserved for stations west of the Mississippi River, and "W" limited to stations east of the river.

This is a relatively new invention as well. When radio first started in the US (late 1890s), callsigns were arbitrary. I believe the Kxxx and Wxxx divide started in the 1920s. Also, this callsign convention applies for TV broadcast as well.

OK thanks. That's cool. Presumably our local radio stations also have them, even if I can't find them with a quick duckduckgoogle.