When I inherited my '26 Coupe, it came with this horn
which was mounted to a cylinder head bolt and had some unconventional wiring, involving (if I recall correctly) a wire connected to the firewall. I think it's a magneto horn.
I understand the normal '26 horn is a battery horn, attached to the water inlet. Is mine a period variant, or something else? I have never seen this form of bracket elsewhere.
Looks like a non ford horn, I've seen them before but not sure what car had them. It maybe an accessory after market horn.
Are there stamped letters on the underside of the flange "Patent Applied Ford" perhaps covered by grime. The bracket has the same worn green paint that our 26 coupe has all over the engine. In that location, which you specified, it could indicate that your horn was originally for an early non-starter basic model 26 T and that a battery, starter, and generator were added later.
My guess is it is a period replacement for the original. Most closed cars came with electric start and charging systems standard. My '26 Tudor had a replacement Klaxon horn mounted mounted on the head with an earlier style bracket. The correct mount should have the horn under the coil box on the Improved Cars. If it works loud and clear after adjustments on battery, keep it and honk away. If not try it on a 12 volt charger and see if that makes it honk...or croak rather. That might give you and idea if it was a magneto horn or not.
Our T was missing its horn but I had a 6v DC horn left over from our 1950 Ford F1 restoration and were able to purchase the 1926 horn bracket to make it work for us. I'm not saying that there is anything wrong with your bracket, only that you could modify it if you desire. We tested our horn on a 12v battery rather quickly but it did not sound right. Later, we tested it on the 6v from the T and sounds good enough for me. Magneto is around 20 volts AC, so you want to be careful with how you test it.
Thanks for the suggestions - I will look for text. I'm pretty sure this is a magneto horn because when I was a young chap I used to drive the car without a battery all the time (couldn't afford one) and it worked then.