Looks like what they probably did was eventually add the prefix "9" to the models designed as monitors, purely a guess on my part though.
I think that this may be the longest running model Altec made(EDIT; 2nd to the A7 of course), from at least as far back as 1965 to as late as 1984. The only changes made were simply the result of the evolution of the drivers and the color of the grill cloth.







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