It's another means for set identification, much like the ACC numbers that we've added to many of the sets. T206 is not found anywhere on the cards or the packaging, but it sure does help in the identification process. Same with the word "Woolworth" here.
-- Dan --
Note: Please see my profile for more info regarding trading (section updated 10/25/2023). I have added a large portion of my inventory to the site, and currently have trading turned off (details are in my profile). I have (finally) unearthed my 2 very large boxes of Star Company minor league sets and they are available (email for details).