Originally Posted by Carcano
https://img.eselt.de/img/18971887_WL29t4BPZMorxT3A/ad.jpg (lower side of barrels, water table with full set of markings, including barrel maker [canonnier] and possibly mechanics / finishers)

https://img.eselt.de/img/18971889_ttEUwuGQfmqanXJD/ad.jpg (partial close-up of above - who is the firm in the rectangular mark?)

Answering my own second question: 😁
Possibly "Société Heurtier & Compagnie", according to a posting of Argo44 here, back in 2016. Barrel makers at large.

https://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=453067#Post453067

And here is a short article about them:
https://fcm25.canalblog.com/archives/2018/05/25/36431738.htm

Carcano