until seeing the pics, i didn't realize these are the "bi-amplified" versions.
Not to rain on the parade, but the model 771 bi-amplifier was probably one of Altec's least reliable and most problematic electronic products. Fortunately, it sounds like this pair have lived their entire lives in good hands, and they may well still be in working order having been spared the abuse of being owned by multiple garage/rock bands seeking concert level output from them.
The other mystery i get from the pics, the aluminum dustcap of the LF driver should be visible through the grill cloth, and it's not.
Also, the low location of the bi-amplifiers suggests these are fairly early examples. Most will have the bi-amplifiers mounted in the crossover network opening just below the "towel bar" handles.