RR, that looks to be the original patent method. Apparently some of these have a corrision problem.

There is a 16ga on GB now that shows separation on the outside. Though the seller seems to be attempting to either hide it or does not realize the problem is there.
http://www.gunbroker.com/Auction/ViewItem.asp?Item=64659356

There is a also a 12ga on GB that is NOT a Pieper Diana, but has had a set of Diana barrels fitted to it.
http://www.gunbroker.com/Auction/ViewItem.asp?Item=64717489

No wonder even some of the better Belgian guns get a bad rap.

Pete