Very true, dates from at least the 1920s, but FWIW, this example is a 1:1 CR, so no compression or low pass filter chamber [air 'trapped' between driver, baffle], i.e. throat area [St] = 8*16" = 128"^2 and is the standardized effective piston area [Sd] of 15" drivers till recent times when differing frame/surround designs were required for [much] increased excursion [Xmax].
How do you know this?
I can't find any dimensions, references beyond it's called an 18" horn, which based on what little we see could be as simple as a 4th order BP with the 'horn' nothing more than its shape when stuck in a corner constitutes a large ~ horn shaped vent, which falls in line with what little I found WRT its performance and frankly is the alignment I'd recommend for such a large driver.