Steve;
First, thanks for the link to the Manton breech plug. The description isn't clear that this consistent of two separate parts (Tang and separate Patent breech?). On the OP build I don't see a definitive "break" for a Manton style BP. Could be there and I just don't see it.
As for the barrel, again....looking at the Rice site the only other barrel I found that's 31" was a Jaeger Barrel--description is for TOTW--However the measurements are close, but the center is not .375. Then again, this barrel is offered from calibers of .45 all the way up to .62.
The first pic is the measurements of the barrel Rice list as "English Sporting".

