The short answer to the practical problem: buy CIP rated cartridges with cases of 70mm or less in length (not 'Magnum').
Stick to a sensible load for the weight of your gun (probably 1oz or 1 1/8oz).
If available, any of the following will work:
Hull Imperial Game
Hull High Pheasant
Gamebore Pure Gold
Eley VIP
Eley Impax
Eley Grand Prix
Kent Classic Game
RST 2 1/2"
all these are made to be safe for se in the gun you showed us, as long as it remains in proof.
Dig, that's a good suggestion for field loads. However, it can be difficult to find foreign-made 2 1/2" target loads on this side of the pond, and the field loads can be pretty stout for volume target shooting. RST is an excellent source for lighter, low pressure target loads for vintage guns.