The HLS are truly amazing.
The HLS have a brass chopper of about three inches in diameter. It is no surprise that pneumatic sirens work very well with small choppers, or ports, for that matter, because when you have air piling up in a big chopper, and expelled through big ports, allot of time and energy would be wasted in getting the pressure of the air flowing through the chopper just right.
My god, is that '273 shaking in the wind?!
The thing that I like about the HLS sirens is that they, or at least the HLS 273, have two stators, one inside the chopper for possibly as little displacement of air as possible, and a sound-function stator on the outside, with passage ways curving down, where they meet with the horns.
Here is a great thread showing the parts:
http://48645.rapidforum.com/area=11&order=u&startid=2, called "Restauration HLS 273."