What is that? 1977S firewall
 

I am half way through rebuilding my 2.7l, 1977 911S Targa. While waiting for parts I started cleaning up the engine compartment (new Bilsteins, new rear sway bracket,etc) and I noticed this on the lower fire wall just forward of where cylinder #3 will be very soon. It looks like a fire bell and sounds like one when you tap on it. Could this be some pre-historic alarm system?
http://forums.pelicanparts.com/uploa...1236823027.jpg

I'm rather sure that's not a factory part. I certainly don't have one on my '77. Are there any wires going to it? You may have to pull some interior pieces to check if it is wired from the inside through a hole.

My car had an aftermarket alarm installed in it (which I promptly removed), but the siren was in the engine bay.

Yep - aftermarket alarm. I found one of those on the front bulkhead of a '70 several years ago.