Bryon, as Kurt eluded, that symbol was included in the date code gang stamp, which was used long before the transmission was inspected. The inspection stamp is located on top of the trans and was a different symbol (see attached for a few examples - 2 inside a circle and 3 inside a square).
So whatever it was, it was done by the person that built the transmission and stamped the date code. It may be something to do with the person that assembled the transmission (in other words, some kind of tracking information on who actually built it), but if that were true, there would be other symbols and we've not seen anything on 68 transmission except the squared off C, a funky looking M (?) and no symbol at all. It's a real mystery since I would assume there was more than 3 people building Muncie transmissions!
Ed