Moving forward, if I’ve linked to this page from my home page, it’s because I have a set in stock, ready to go. And the audio file (below) will be of the actual set of horns I have for sale. (Most of the information on this page comes to us via the University of Michigan. The entire story of how the 1929 version of pitched horns came to be known is told here: Gershwin Taxi Horns.)

A3 D5 Bb4 Ab4 from left to right. The DCBA refer to how the use of the horns was noted in the score (Not the tones to be played!)
The 4 horn set comes with 4 attaching brackets.
This set of horn’s pitches.
Compare to taxi horns used in the 1929 recording George supervised.