Answers to questions:
1. By law, electronic "air horns" cannot be used in CA if they interrupt wail or yelp; it would need to be a separate electronic siren with its own speaker(s). This would cost as much as the horns.
LAFD did extensive testing of horns and sirens, and determined that the hi-lo horns, when used with a siren, allowed the EV to be heard better than when sirens only were used. The tones are lower, so they seem to help people who may not hear a high pitched electronic siren. Calif. law also allows an air horn (type not specified) to be used on fire vehicles. I read that as air horn, not simulated air horn. The horns have been used since around 1972 or so.
2. The horns are not Martin Horns; they are Stebel for LACo and Fiamm for LAFD. The Stebel use non-Stebel trumpets (Stebel use a composite trumpet pair, and these are metal), and the Fiamm sets use Fiamm trumpets, either 2 or 4. LACo typically use somewhat higher frequencies than LAFD. That is a matter of preference. I do not know who makes the LACo trumpets. They are adjustable, and I have seen 4 used, adjusted to produce a tremolo, sounding exactly like the Martin 2297. The seller will not reveal who makes them.