Congratulations with the Rigby.

I have a similar one, just a hammergun with jones underlever.
Mine has a serie number very close to this gun.

Mine starts out with .742" and ends up with .735" at the muzzle.

These Rigby B&S were proofed af rifles what equalled to 3drams Black(powerwise). Ours just with similar smokeless.

My barrel has 3 indentments which is clearly visible if one put a .740" roundball in the barrel.

MIne has a slight higher serienumber. My was made in 1903 and bought in 1905 by a James Jennings(he wrote a book).

As stated these Rigby`s ball & shot are very very rare. I actually don`t think they were popular...bot some bought them like you and me.