This is a generic "Closed Captioning" symbol, which was created by Jack Foley, senior graphic designer at Boston public broadcaster WGBH, which collaborated with several others in the earliest demonstrations of closed captioning for television. In the early 1980s, the only available symbol to indicate a closed captioned program was a proprietary symbol developed by the non-profit National Captioning Institute (NCI). NCI only allowed the symbol (the "TV with a tail") to be used on programs they captioned. In view of the need to have a symbol that WGBH's Caption Center could use on programs it captioned, this symbol was designed (the "CC in a TV"). In the mid-1990s, WGBH released the symbol into the public domain so that it could be used generically by anyone who was closed captioning TV programs.
http://main.wgbh.org/wgbh/hire/symbols.html
WGBH