Posted by Bruce Kahl on October 07, 2005
Someone emailed me an alternative to what I found and it looks good:
"Bruce - Before the days of plastic ball/strike counters, the umpires used knotted strings. The string in the right hand (strikes) had two knots in it, and the one in the left had three knots. On each pitch, the umpire advanced the appropriate string one knot. So, on a 3-2 count, the strings were "out", shortened to "the string is out"."