This may be old news to some of you, but I had never seen anything quite like this before and could only find one posting that was related, though with a different maker and with a different style.
The gun is made by W&C Scott & son for JP Lower (Denver, CO), and is currently up for auction. I would love to see what it looks like when cocked, although to my eye, the curve of the 'hammer' is backwards.

Looks to me like it's not a true hammer gun. I have one made by Henry Adkin (Bedford) c.1880-ish, a rotary underlever with a straight stock and Damascus barrels. They are considered evolutionary, another step towards the modern hammerless profile. I think it is one of the most fascinating phases of the development of the modern gun. The articulation of the would-be (exposed) hammer serves to cock the interior (true) hammer. Mine goes to half cock first and then to full cock. Gil

Here's the other thread related to these semi-hammerless guns. https://doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=266821
Definitely fascinating, although I don't find them overly attractive personally. I guess the idea was that you had an oversized cocking indicator and a way of uncocking it without snap caps.

Reminds me of the Iron Pig FJ55 land cruiser... couldn't figure out if it wanted to be an FJ40 or FJ60 and got stuck somewhere between.