For those interested this is the famous Rigby Sidelock incorporating
the Rigby Bissell Rising Third Bite action. The "loop" of metal on the
end of the barrels is a dead give away for a Rigby Bissell Rising Third Bite action.

I will try to get a photo sometime of the actual Rigby Bissell Rising Third Bite
and have it posted.

Not long after this, Rigby started to use Webley actions for the
sidelock guns they made as well as the Boxlocks.

500 Nitro