Note the braze lines are quite visable in the picture on eith side of the under lug where the lower part was put in. Since it wasn't built as a monoblock, it doesn't have one now.
Monoblocks have the entire breech end of the barrels, including lugs etc all machined from "ONE" (Mono) piece of steel.
A breech section with joints in it ain't Mono.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra