1-46, which I was very surprised to find. It seems with a twist that slow, it would only be compatible with the smallest 150 gr bullet this cartridge was loaded with. To my modern sensibilities it seems like it would even be too slow to stabilize that bullet properly. 1-46 would make a fine round ball twist though.

This is why I am trying to find out what twists other might have in their rifles. So far it's looking like anywhere between 1-15 to 1-18. Hopefully Curl has a chance to compare what is in his Rigby, which would be an excellent rifle to compare it to.