I was going back trying to remember what the source was of the name "550" or "550-AT" for the dual-headed, rotating, dual-tone Federal siren, such as in Calhan, CO. The reason I ask is that I have something mentioning various warning devices (without pictures) and one of the listings is:
Federal Sign & Signal Corp. 500-DHTT Horizontal axis, bidirectional, 670 & 450 cps
DHTT would presumably be Dual-Head (DH), Dual-Tone (TT). The listed frequencies work out to essentially a 12/8 port siren, which is what the Calhan, CO siren sounds like. "TT" and "T" seem to be naming conventions used throughout various years by Federal. Most literature I've seen uses on "T" for dual-tone, but I have seen the "TT" before. Notably, Dane County, WI lists some of their sirens as "T" and some as "TT". Do any of you have more information on sirens like the Calhan, CO one being referred to as either a 550 or 500-DHTT?