It is a ventometer. Various types were made, to allow the fine adjustment of rifle sights.
They are collectable items, and a bingle of 'ventometer' then clicking the 'images' tab will show you all manner of them. Peter Dyson has a few for sale.
Here's an illustration of one (not the same as that from Mr Murcott) together with the rifle concerned.
There is a very nicely finished T Murcott gun on sale somewhere; damned if I can remember exactly where that is. The 'Mousetrap' business was supposed to be a bit of negative PR from his competitors, but having handled one I can confirm that not only does it make a steely snappy shuuting noise it also can trap an unwary finger in a most unpleasant manner. Still, you'd only leave your hand there the once, wouldn't you!