My Pieper hammer gun marked Modified Diana has the tubes threaded into the Mono-Block.
Pieper's original patent which results in the "Step" at the end of the block had the tubes inserted from the breech & wth a taper. Many Piepers one sees built with this construction are marked Original Diana. Not much posssibility of tubes with either of these assembly methods moving forward.

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