Thanks David, for additional info, it is a fascinating object. As I mentioned in first post, the item itself is certainly original, no question. I went to link you posted and got something about "session killed." I then sent to main museum page and searched getting as far as the basic data here:

http://vhs3.vahistorical.org/cgi-bin/starfinder/6650/p.museums.txt

but no additional pix. I would like to see the other side of it with the WD and broad arrow marks. I have a lot of friends in the UK antique firearms area and with a bit more info I should be able to get an ID for you.

I'm curious as to what the function of the conspicuous steel? nub-like projection below the barrel is, does it move, as some kind of safety, or is it for alignment with a mating object perhaps?

The Brits made an amazing assortment of devices over the years, many like this one quite finely-crafted, but at this point I can't tell if this is a signal device, fuse-lighter, or "humane cattle killer." The latter is of course not in the running if it has authentic British ordnance property marks.