Rich,
you have partly hit the nail on the head about that horn.
I have a thread running on
www.hmvf.co.uk about what is the longest continuous usage of a British Military vehicle part.
This item, is one of 3 contenders that I have identified. The others being the venerable convoy light which is still used, and the red rear reflective disk that came in post war.
Currently that horn is still used on active service vehicles such as CVRT and 432 and Stormers.
It is an incredibly long time for a single part to have been used wouldnt you say?
Robin