I am very grateful for your prompt and informative reply. Well, next I need to contact the curator of the Museum where the shotgun is exhibited.

There are several other shotguns on display. Would appreciate it if you provide information on the following:

http://blog.sina.com.cn/s/blog_3ec4d3090101fmcv.html

The most curious thing about this shotgun is that it bears HOLLAND HOLLAND on its right sideplate (cannot see left side) but H&H confirmed me that it is definitely not their product.