I found a Federal Siren marked Q2BS at Carlisle this spring, and what makes this one strange is that there is no rear cover on the siren, and it does have a rear band cover with a data plate on it that reads Q2BS... I know that what the Q2 is, and the the B refers to it having a brake. I have never seen one marked with the "S" designation. Also the mounting base is different than any others that I have ever encountered. I am thinking that this has to be an early siren, since it has a full screen behind the "F" on the front cover...