'ACCI' was a mark of Syndicat des Pièces Interchangeables (along with 'SPI') established in 1899 by Neumann Frères, Janssen Fils and Dumoulin Fils & Co.
https://www.littlegun.be/arme%20bel...des%20pieces%20interchangeables%20gb.htm

[Linked Image from photos.smugmug.com]

Rather casual attention to fitment of the barrel lump

Odd bore at 16.0 = .630" but that was probably at the time of First Provisional Proof of the tubes

Please tell your friend to clean up the exterior surfaces, squirt epoxy in the firing pin holes, and hang it on the wall. I suspect the ? dovetail lump separated at some point.
A pic of the barrel flats would be interesting.

https://sites.google.com/a/damascusknowledge.com/www/home