Bill:

With any pre-war Brit double that is new to me, I'm certifiably paranoid. Not so much from the fear of blowing it up...I just HATE the idea of abusing them, and want to be certain that what I do is right.

Are the marks on the flats "Cordite 43, 310 MAX"? Is there no other information on top of the barrels? If this is the case, my operational hypothesis would be that it is a .400/.350, not a No. 2. Next, get off an inquiry with the Rigby serial number to Paul Roberts at J. Roberts & Son in London. Tell Paul what you have, and what you're trying to figure out. Yes, Paul sold Rigby in 1997 and no, you can't get any data out of the new owners. However, Paul kept photocopies of the Rigby records. The data in the records may specify which .350 it was delivered as, if you're lucky.

Yes, cast the chambers long enough to get all of the leade. Slug the bores. Next, find some original Kynoch .400/.350 ammo. Search the net. Definitely check with Lewis Drake. Dismount the barrels, push the extractors all the way in, and see if a shell will drop all the way in with no additional assistance. If so, since the rifle is proved for that shell, it should be safe to fire it in. If the barrels shoot wide, it may have been regulated for the No. 2. If the shell won't drop all the way in, it may be a No. 2.

If it looks like it may be a No. 2, I wouldn't pull the trigger on a No. 2 round until I was as absolutely certain as is possible that it is. Have a conversation with David Little about it. Again, TRY to find some original No. 2 ammo and fire a few rounds of that first. Proceed with care.

If this rifle is what I suspect and the barrels are in good shape with nice bores, it is worth correcting it's problems properly as long as the price is commensurate with the work required. Most Rigby boxlocks were Webley's which are widely regarded as among the best boxlock DRs ever made. Tain't junk you're playin' with.
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