8bore - That is a seriously nice rifle. Even if not dated it has all the hallmarks of an early rifle, non-rebounding locks, finger rest etc. and the engraving is my favourite
I saw a very similar Purdey with a similar trigger guard/underlever some years ago at Holt's auction in London

If you are loading BP you can use milk carton wads between a grease cookie (Daryl has a good recipe)

If NfB with H4198 you must use magnum primers

I now always use BP as that is what they were designed for
I have an early Williams & Powell DR in .577 Snider posted here a long time ago
BTW .577 is 24 bore

Good luck shooting it please keep us informed of your progress

TH44