Bit of a bump, Brendan W found another 500-DHTT at a high school while we were hunting for sirens in Yonkers, NY
https://goo.gl/maps/TCDjAWbucvP2
History about their Civil Defense System:
Sometime around 1942 Yonkers had purchased 29 2-horsepower sirens and placed them throughout the city and atop schools. They noticed having coverage gaps in 1957 around places in the city. Around 1958, they got a contract with Herbert G Martin to install 15 20-Horsepower sirens to eliminate the coverage gaps.
Over the next year they would install the 500s on galvanized steel towers, at schools, on top of buildings etc.
They discontinued Civil Defense siren testing around 1976.
Special thanks to Brendan W.
I will add more to this post when I find more post worthy information.
Edit 5/5/18: I took pictures of both the Untermyer Park 500-DHTT and the one at Gorton High School, the pictures are on the Facebook group and my Flickr.