The proof mark I can’t identity is the JA8C under a crown. Then brevete mark as I understand means it was built under permission of the patent holder, who ever that maybe ie colt brevete
There was so much copying going on in Belgium, it was nearly impossible to track down every manufacturer. The major arms patent holders went by the expedient of having inspectors stationed at the proofing house, looking for guns coming in that were obvious copies of existing patents. When they spotted something, they would inform the Belgian officials, who would notify the manufacturing house, who then would pony up a nominal licensing fee. No fee, no proof stamp (which would make it hard to sell the gun), and no licensee stamp.
The licensing inspectors worked on a commission, getting a % of the license fees they caused to be collected. Good business all around.
I think what you're seeing is "JA
& C" - not "JA8C"
So J (possible A) and Company.
Going through the list of identified Belgian manufacturers, here:
http://www.littlegun.be/arme%20belge/a%20a%20artisans%20identifies%20gb.htm, I couldn't find a match in the J's, but that doesn't mean much. tons of unidentified craftsmen, and even the known craftsmen, not all their marks are known.
If you have an entire day to kill, you can go through all the marks listed at that site, and you might, or might not find a match.
It truly is staggering, the number of Belgian gun makers at the time. Many of who's names are lost to history now.