When CSMC brought out their 16 ga barrel sets for the Parker Repros ($1895 plus fitting?), it seemed to take a long time to move the merchandise. That's with a gun is is plentiful (relatively) and owned by a lot of people with disposable income (at least until recently). CSMC also had a bunch of aftermartket barrels for the Win Mod 21, and those wound up in their closeout section. There was a shop discussed on the Parker site (among others) that claimed to have original barel tubes or complete barrel sets for Parkers, and all the hype never seemed to produce a satisfied customer, even though posters kept asking if anyone had received a completed/fitted barrel set.
I have three Parkers with aftermarket barrels. One has Bohler barrels with vent rib and seems to be up to Parker standards. The other two have unidentified Italian barrels and, while nice, are not up to usual Parker standards. There's a lot of variation out there, and thus a lot of risk to the buyer.
The barrel sets I'd really like to see would be 30" fixed choke from CSMC for the RBL 20.