J Henry, you have posed an interesting question. Just what function do the fins serve on the 811 (and 511) horn?

Certainly they are not for pattern control, and being obstacles they would degrade the pattern slightly if anything. Perhaps they were intended to stiffen the structure, though the horn is still plenty stiff without them.

I think there might be other reasons: styling and product differentiation. The first version of the 811, the H-811 or 1952 or so, was a stylish thing. It was a one piece casting with a curvy appearance to the mouth, and the fins gracefully arched outward. It must have been a bear to cast, as it was replaced within a year by the two piece welded 811A.

The RCA MI-9594 (60 degree) and MI-9595 (90 degree) cast aluminum theatre horns had been introduced in about 1950. RCA produced the first radial horns, as the radial horn had been invented by RCA engineer John Volkmann. The Altec modified radial horns were obviously inspired by Volkmann's work. Perhaps the fins were intended to differentiate the look of the Altec product from the RCA horns, which did not use fins.

Another reason may have been to maintain a visual continuity with the earlier Altec horns. Altec Lansing had produced only multicellular horns until the introduction of the H-811, and the fins served to visually divide the mouth area, providing an evolutionary appearance change rather than a revolutionary one.