In the 70's and 80's it was very common to see specifications that said "Panelboards shall be Type NQO - or approved equal - as made by Square D, General Electric, Westinghouse, and Cutler-Hammer".
In reality NQO was a type of Square D panelboard, rated 240V max, that used QO plug-on breakers. Its sister panel was the NQOB, which used QOB bolt-on breakers. These panels were replaced around 1990 by the NQOD one, which accepted both style breaker. The current generation of panels is the NQ.
For the trivia buffs, Square D has used the prefix letter N to indicate a 'lighting and branch circuit' style panelboard since the 1940's, the letter Q has been used to mean 240V max equipment since the 50's, and the letter O probably started out as a number 0, since there are breakers numbered Q1, Q2, and Q4 as well as QO.
