A spangled and fused AC is very likely an inspector's mark, and there are others that are not shown here, like this fused AV

[Linked Image from photos.smugmug.com]

Unfortunately, related to a fire at the Belgian proof house some marks are not recorded, and others were repeated (as shown) so can not be used to date a Belgian shotgun

[Linked Image from photos.smugmug.com]

Pete Mikalajunas' site is no more, but I was able to find and save his extensive list of tradenames and maker's marks.
There no AC in his list.

I could be of more help with a full size close up of the barrel flats and marks just forward of the flats.

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