I just spent some time poking through the BMW Mobile Tradition Archives, but couldn't find a really good photo of a late series R11 horn. It's clear that the early models -- those with either the drum or the teacup headlight -- had the old style Bosch horn, which has a perforated grill and a megaphone on the front.
But it seems that the late model R11s, with the later, larger headlight bucket, have a horn in the style you show. As I said, I didn't see a photo that displayed the front of the horn, so I can't say what is correct.
Do you have a parts book for your bike? Is there a drawing in there of the correct horn?
But I have seen some pictures of serie 5 with the horn resemble to the one in the parts book. (see the picture of a serie 5)
I think the horn I have is a prewar horn, becouse my projectbike was placed in a old house after the ww2 in Norway and I don't believe that someone have been there and change the horn. No indications on the projectbike.
Do some of you recognize this type of horn?
It was placed on my Bmw R11 serie 4/5 projectbike when I got it.
I don't know if it's a German horn at all...