Thank you for the comments chaps.

500N

The "loop" part of the rib is both mechanically interlocked at its forward end and soldered into place. The mechanical strength is greater than the shear strength of the C part of the loop.
What you can see looking at the breech end at the moment is a set of dummy ejectors that are there to prevent rounding over of the ejector grooves whilst putting the gun onto face. The actual rib is one piece that extends from the loop to just after the unfinished section of the breech end of the barrels toward the muzzle as you look at it now.
If you go to the website here :

Defunct website
you can see the rib in position in the last photo.

All of the guns have disc set strikers. What you can see on the standing breach are machining reference points. Once the gun is fully on face we spot the correct striker position with a precision ground "dummy cartridge" that has a sliding center punch. ie we load it into the chamber, close the gun and tap it with a rod down the barrel.
Thus if any minor variations are caused in the hand fitting, when the strikers are finally drilled from these points they always correspond precisely with bore center.

Andreas

The barrels you see are the correct but yet to be struck up and finish chambered chopper lumps.

All The Best.