I don't know of any personally but if you would post here, you would probably find out about any in public or some private collections in the US. You can post there as a guest using the colored button at bottom of the page here:
http://www.military-historians.org/The site is currently limited to one photo, but you can insert a link to another, or simply link back to this post if you feel all 3 are required.
I'm glad you posted the photos, by the way, because I've never seen one of those horns but would be tempted to buy it if I thought I'd found a real one.
Did you see my recently-acquired model of a British traversing platform, here?
http://www.go2gbo.com/forums/index.php/topic,185849.0.htmlIt probably belongs in some UK museum. Maybe one of them has an ugly old unwanted Whitworth 12 pdr. RBL to trade?