Though it resembles a 550, and I believe it IS a Federal siren, it is not a 550. When Adam posted this picture a few years back I'm pretty sure that someone on here posted its model number. Whether it was correct or not I don't know. So far I've been unable to find a video of a siren just like this one on YouTube.
The center intake of the 550 doesn't protrude outside of the projector assembly like the siren in the picture shown, and the projector of the 550 has a much wider opening.
Go check out the YouTube videos of the 500s and the 550s and you will clearly see the differences between the true 550s and the siren in Adam's picture. Similar, yes, but not exact. If memory serves me right, the siren in Adam's picture was introduced years before the 550s came out.
Adam, if you happen to read this post, and maybe you were already aware of this, but there was a T-Bolt located almost across the street from the fire station where you took the picture of this dual-headed siren. That T-Bolt was blown down and destroyed by the EF4 Good Friday tornado this past April 22nd. The tornado actually barrelled down Fee Fee Road, where this fire station is located, and firefighters and civillians took shelter in the hose tower of the fire station.