I remember reading somewhere that it was a 10 port single tone Klaxon siren on 60 Hz. It peaks at the right pitch for a 10 port siren running around 3400 RPM. Can't be a Model 2; it has the linear wind up of an induction motor vs the Model 2 which has an exponential wind up being driven by a universal motor. IMHO if someone is going to make a synth like this (amazing!) at least use the right type of wind up... Here's a full length video of the siren: https://www.youtube.com/watch?v=N-sEmR0yPOE You can also hear a kid scream at around 49 seconds in. I'm leaning towards it being a true recording, because you can somewhat make out the pulses made by the chopper and the fact that the 10/12 port siren continues on in attack. Listen closely to it with some headphones. It sounds a bit like a H.O.R. Siro-Drone, compare it to this: https://www.youtube.com/watch?v=mzmwtb71VZM I've read (keep in mind on YouTube) that the 10/12 siren was anything from a 2T22 to CLM. I think a rotating CLM wouldn't be all that bad of a guess, as the siren in the background sounds like its 12 port chopper is dominant to me, and it sounds like it has two choppers as opposed to one.
*Credit to the video owners*