Good! My next guess:Juster, there are no gas ports to bleed off the gasses to work the action.
I think Juster has it. It looks like the "attachment" allows the rifle to be mounted coaxially with something. Either as a sub-calibre device, or perhaps to allow remote aiming.
It looks like a mount for use as a sub-calibre device. We used FN's with .22 sub-cal inserts which were mounted on our 105mm howitzers to punch 6" x 6" paper tanks at 25 meters.![]()