The seller is calling it a Howdah.
https://www.gunbroker.com/item/893542529

Its the first swivel barrel version Ive seen. So it essentially a single shot with a fast/faster second shot after some manipulation. I guess that better than an actual single shot but hardly as fast as a true double. Of all the Howdah designs I have a secret lust for an O/U but I wouldnt be satisfied with "just" an O/U. SxS's clearly have the "look", and if it wasnt for the O/U that Remington carried in "that movie" I probably wouldnt feel the lust.

500/450 No. 2 Musket seems an odd cartridge; but maybe if you were carrying a similarly chambered rifle, it sorta makes sense???

Ultimately Id buy it if I had the $$$. Is it a good price IDK; but I bet if it was called a swivel barrel pistol instead of a Howdah, then the price would be a few thousand less