It is definitely made for black-powder pressures: that "3-1/2 TONS" is the working chamber presure in Imperial Long Tons of 2240 pounds each: 7840 psi.
The action is a Martini type, so it will take just about anything (within reason). It is likely that the gun will handle more pressure than what the Proof markings show, but it is best to stay on the safe side with something this old.
Can't read all the markings very well. Be very careful with chamber depth.
As to brass, Cabela's are the agents for the excellent Brazilian-made MAGTECH brass shotgun shells. Their brass is 2-1/2 inches long, comes 25 in a box for $22.95, can be mailed and reloads easily. It is available in several sizes, including 12, 16, 20, 24, 28 and 32. REALLY wish they would make 10 and 11, but, no such luck. They do look after your gun, though.
Hope this helps.
.