That action was used by hundreds of makers in Belgium and France, and, perhaps Eastern European makers for most of 100 years. They are not rare, not truly valuable unless finished to an exceptional standard, but, are very useful bird guns. I would expect to find good wall thickness, as it was proofed in St. Etienne, tight chokes, and light weight.
The 2 3/4” chamber modification needs to be done in a thoughtful manner, with a double gunsmith measuring the bores, noting bore condition, and utilizing precision equipment and professional expertise. I would recommend Dean at Skeets for that work, but there are others. When I started in this game 2 1/2” ammunition was challenging to come by, but, that has changed. I usually leave the chambers alone, at this time.
Good luck. I hope it works out well for you. Mine have.
Best,
Ted