I heard the same story about the Charles I Anderson music company, and I think I even know who improvised that name when he forgot the facts behind the story. Nevertheless, one of the members of the Tech Square research community at the time asserts the CIA office was listed in the directory as "R K Starling Associates," which is at least slightly less obvious:
Before the CIA office on the third floor had its typewriters stolen one night, the MAC building was open all the time, like a university buliding. In the evening, newsboys (Cambridge urchins) used to run through the halls at Tech Square shouting "Ecket" — selling the Boston Evening Record. The building directory listed the CIA office as "R K Starling Associates." After the theft, there was a security guard in the lobby and you had to sign in and out.
http://www.multicians.org/tech-square.html
Oh — and the banner's letters are blue.