Texinfo ultimately gets the @ convention from Brian Reid's Scribe[1], as developed at Carnegie Mellon during the late 70s and commercialized by Unilogic[2,3] in the 80s. Coincidentally, there was a close derivative of Scribe called Mint[4], also developed at Carnegie Mellon in the early 80s for the PERQ (an early personal workstation competing in the category of things like the Sun-1 or Lisp Machines).