This is from the description Full Test of Custer County/Miles City, Montana Outdoor Warning Sirens. This one is the only one that is not a STH-10. Notice its 3 independant motors.
It's definitely a one of a kind siren, whatever it is. Also, it takes a while to wind down, indication its a fairly decent sized siren...much bigger than a model a or possibly even a model 2.
This is probably one of my most favorite sirens, I wonder what the chopper ratio is on that thing. It appears to be timed to sync with the other motors.