I must take exception with the inaccurate information posted. A called third strike is denoted by a straight K. A swinging third strike is indicated by a backwards K. The backwards K symbolizes the position a batter would be in when swinging, and missing; hence the backwards K…
Welcome to the Straight Dope BklynNY. It’s customary to include a link to the staff report in question. Let me do it for you: In baseball scoring, why is a strikeout marked with a K?
Um, welcome to the SDMB. 
But Wikipedia:Strikeout disagrees with you. There, they specifically note that a strikeout looking is denoted with the Backwards-K. See also Armchairgm, and, very importantly right now, RedSoxDiehard.com. So with respect, I think your story is not accurate. Nor, frankly, does it make sense, since it is dependant both on the direction the batter swings from (left or right) and whether you are behind, in front, or towards the mound from the batter. 
I’ve never heard of anyone using the backward K for a swinging strike. Backward K is for a called third strike. Unless you didn’t see it, then like Rizzuto you put down “DSI”.
Hey, the poor guy’s from Brooklyn, so he hasn’t seen a baseball game in 50 years! No wonder his memory of it is beginning to fade.
–
John W. Kennedy
(whose mother graduated from Erasmus Hall)
This is correct.