I have what I believe to be the correct horn for my NB - the Lucas New Alto. However, it has been mounted inside the engine bay and is missing its domed rear cover. If anybody has a rear cover available, I'll be interested in buying it.
I believe Keith Ardley makes new ones tel07957252421 I have a s/hand one with afew small dents and could possibly use a rechrome £5 +postage or collect from Stoneleigh Terry W.
I believe there is one on eBay at the moment..... The seller is advertising it as a Jaguar horn, but I think it might have also been used on the N-Type as well.