Old boxes like these probably appeal to the 2nd Harmonic crowd.
For low E (41 Hz) the 2nd harmonic is 82 Hz.
The shortest quarter-wave horn path that can produce 82 Hz is 41.26 inches.
The shorter the path, the higher the low frequency cut-off.
A square mouth 41.26 inches on each side will fully produce 82 Hz.
Other than a trip down Memory Lane, these boxes are totally obsolete.
There is no good reason to use a 15" driver in a bass horn.
Rear loading horns where the driver front is open to the air, are relatively poor performers.
Yes, JBL made the [Invalid or Expired Link Removed]. No, JBL no longer makes these.
The JBL 4530 is a quarter-wave horn to 40 Hz.
It weighs 120 pounds without driver.
Response drops like a rock below 60 Hz.
Other than nostalgia, those wanting a bass horn should consider a modern design.
The Titan39 and Titan48 will both snuff any of these old designs, and do it with less weight.
The Punisher horns cater more to the 2nd harmonic crowd, with their higher cutoff frequency and shorter path.