Ips, already have done mate. They've no idea either why the ribs are solid. The guy I spoke to said they may of been been ordered that way.
The barrels were either ordered like that, or they're from a 687, which I have my doubts about. The only 2 '600' series that share barrels with replaceable shoulders and Optima chokes are the 687's and 682's. ( to the best of my knowledge anyway ) the 686s had the Mobil choke system until the Evo came about.
The only 687's I've seen with replaceable shoulders have vented ribs. Which were sporter barrels anyway ..
From memory, the barrel serial numbers on my previous Berettas were stamped under the bottom chamber, as they are on my latest 682, using a dot punch rather than a letter/number stamp, as they are on the gun in question.