OK now you got me you told us why we got dog tags and where they came from BUT what about the Notch ?? what is it for ??
The notch was/is merely a mechanism to ensure the “correct” side was stamped by the machine, nothing more, nothing less.
Welcome, I337 h0b0 . If you could include a link to the original article, it helps. The original SDstaff article by Songbird was What’s the origin of army dog tags? .
And, as Tedster correctly replied, the notch was to hold the tag in the machine that made them. Up until the 1970’s or so. Since then, they don’t have notches.
tomndebb first posted the authoratative answer about this misconception/urban legend back in 1999. This link to that thread is http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=2173&highlight=tags+notch