In bass horns you can design a throat size without a plate and hope for the best, or design the horn, build it, and then use a constrictor plate as a fine tuning apparatus. I'm not positive about the Altec situation, but in the case of Klipsch that's exactly what happened. Paul Klipsch wasn't satisfied that the KHorn worked as well as it could, so he tossed in a restrictor plate to see if it made the horn work better. It did. It's a far easier process to play around with various restrictor plates than to rebuild the horn in a dozen different configurations. It's a very safe bet the Altec plate size was arrived at the same way.

There are advantages to smaller throats, primarily in that they can give better HF loading, but they can also take the Fs(h)[driver/horn system resonance] too low. A constrictor plate is a simple but effective way to adjust the Fh(s)without major surgery.