I discovered a couple of days ago through a YouTube video of a Whelen 2910 monthly test activation (siren located in Chesterfield) that there's a Federal STH-10 in St. Louis County's siren system. At the end of the 2910's test, the STH-10 can be heard wailing away in the distance, just a few miles away, though it sounds sick and is very low-toned. I had no idea that this siren even existed, and even more surprising is that it's still activated by St. Louis County E.M.A. and never taken offline when the massive new system of Whelen 2900-series sirens were installed in 2011.
The STH-10 is located at St. Louis County's water treatment plant in Chesterfield, at the Missouri-American Water Company building on Hog Hollow Road. I found a picture online of the water treatment plant and, sure enough, the STH-10 can be clearly seen, mounted on the roof of the office building. It's painted gray instead of the familiar "CD Yellow" of the old St. Louis County Civil Defense siren system installed back in the 1950s and '60s. I have no idea how old this siren is, when it was installed, etc. But it doesn't have the typical high pitched tone of a "healthy" STH-10 and is, instead, low-toned, like the motor is burning out.
The building itself is old and I'm assuming that the purpose of the siren being installed there was for CD use and not as some sort of warning siren in case of an accident at the plant. It would be interesting to know the STH-10's history.