It is a percussion muzzle loader.
21.0 mm would be .827". 8 bore is .835". .875" = 22.2 mm and would be 7 bore.

As Raimey observed, the crowned inspector's marks indicate production 1853-1877

The other proof marks. Le Perron should be somewhere

[Linked Image from photos.smugmug.com]

I suspect the rib mark is "REAL TWIST"
'NM' is not uncommon on Belgian Twist barrels but unfortunately, we haven't been able to ID the maker.
It is possible that the gun maker's initial would be on the inner surface of the locks.

Glover Cleveland was an avid sportsman. This image is likely him with his 8g Colt hammerless double, the only one produced.
Courtesy of NJ Div. of Parks and Forestry, Grover Cleveland Birthplace State Historic Site.

[Linked Image from photos.smugmug.com]

https://www.wideopenspaces.com/take...r-clevelands-giant-8-gauge-colt-shotgun/

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