Look like early Brit as you say, but remember Rosier was an Australian colonial "gunsmith" and may have nitro proved earlier guns here in Oz where there has never been a proof house as such, and no proof laws AFAIK. NP stamped on shotgun barrel flats can only mean nitro proof.

Also, no period-original black powder .410 shotshells are likely to have been 70mm long, which I assume is the chamber length.

Just my opinion, but if the gun is fairly robust in dimensions, and not stupidly petite, (and still on the face!) I would give it a go with Fourlongs or equivalent, but probably steer clear of Winnie Super-X due to the age. Modern Eley plastic-hull .410 shells may be a bit full-on too!