The siren on top of the Round Hill VFD is hidden inside a cupola. I hear there used to be an Allertor there, but I never saw it and am assuming that the Allertor was part of DC's old system. I don't think you could fit an Allertor in a cupola, so my guess is that it is either a Model 2/3/5/7 or an STH-10. But, many of the firehouses in Loudoun County used to have Allertors (some still do), so maybe it is an Allertor for all I know. Could you fix an Allertor in one direction and put it inside a cupola? In your opinion, what kind of siren is it? I have never heard it myself.
Street view: https://maps.google.com/maps?safe=off&q ... =0CJgBELYD