Muzzle loader percussion SxS double barreled 12 ga shotgun with 34 inch barrel. Made circa the 1850s in England.
SxS - Side by Side.
Percussion - The percussion cap ignition system was developed around 1818 and replaced flintlock firing. The percussion cap contains a small amount of fulminate of mercury. This cap is placed on the nipple and when struck by the hammer the fulminate of mercury explodes sending a hot flame into the powder charge inside the barrel.
Percussion firearms were the dominant technology until the rise of the modern cartridge in the 1860's...

An appraiser told me it was a London Fine Twist, but it doesn't say London Fine Twist anywhere. It looks like a London Fine Twist but has "W. & C. S ...." on the side, which is William and Charles Scott.
The barrel markings are also "Birmingham Proof" not "London Proof", and indication of a W&C Scott, not London Fine Twist.

W & C Scott were one of the major English gun makers dating back to 1834.

A lot of guns marked "London Fine Twist" are fakes. A W. & C. Scott gun similar to this sold for $260 at a 2018 Horst Auction.
Accoding to a 2018 article, W. & C. Scott Archives - at Dogs and Doubles,
"Back in the '90s, most hammer guns were curiosities few people were interested in. But thanks to magazines like The Doublegun Journal and organizations like The Vintagers, these old SxSs became the must-have doubles everyone wanted. Old ones that wouldn't have brought $500 in 1995 were selling for $2,500 by 2005."

How a muzzle loading shotgun works. - YouTube


Articles - California Muzzleloader Hunting Season