On several occasions I noticed that the brass horns on 13-14 cars were painted black with a bare brass rim. Is this correct? I have an original Rubes horn I plan on putting on my 1914 touring. Should I be painting it black? It looks like there are traces of black paint on it.
Yes, it should be painted black. The screen and its rim are bare brass.
The best site on the internet for original period photos of 1914 Model T's is here:
You will se hundreds of horns in the photos, all of the pictures taken at the time the cars were new, or nearly new. Not one photo shows anything other than what Mike Walker describes.
Looking through the pictures I noticed that some engines sitting in the chassis were unpainted or grey and some were black.
Don't think any Model T engine was painted grey.
All were slush coated with black wash. An engine in one scene that appears lighter in color also seems to be reflected bright light from the photographer's bright camera lamp, as even the workers clothes are bright.
The 1915 Ford Methods and The Ford Shops clearly lists the step for the engine paint, which is black. But a thin coat and this type paint wore off rather fast in use.