I took a second look at enlarged versions of the pictures you provided ad it looks like there may be a reduction in the diameter of the barrel similar to what would be if the end was threaded.This of course creates a shoulder. The hole in the barrel plate is sized to fit the barrel, or vise versa, and the barrel is retained by the shoulder on barrel and the wedge shaped clamp. Am I seeing this correctly?
"You are seeing it correctly. The shoulder is what stops the barrel and holds it under pressure from the wedge and it's clamping screw that is tightened to the barrel plate. The wedge relief cuts in the barrels need to be in sink with your extractor relief cuts also.
I used a 25/64 ( .3906 ) reamer for the barrel plate, then turned
the barrels down for a slip fit.