Here is some info on the sirens that the ROC Posts were equipped with.

"All posts were equipmed with a hand cranked Secomak 447 Air Raid Siren. There was different tones for different warnings. The tone of the siren could be changed by twisting a clamp on the top of the handle and from winding the siren at a different speed. The siren was stored in a crate in the post and when not in use could fold down smaller.

Here it is packed down.


Air Raid Siren (Packed) by Krypton-UK, on Flickr

And here it is assembled.


Air Raid Siren (Assembled) by Krypton-UK, on Flickr

They really do produce a great sound!