MN,
The last set of barrels I saw for sale had an asking price of $3500. I missed a set some time ago at William Larkins for $850. I believe they made them fit an SO5 they have listed as a two barrel set. I talked to Rich Cole about this idea last year and the only sure way to add a barrel was sending it to Italy. The price would be eye watering. The SO's are not great collectors in general so in this case a set of Briley thin walls would, as already stated, be a very nice way to go.(IMHO) You made a great purchase to be into the S2 at $2000. The two listed now are about $4900. William Larkins had one and Cabelas had the other.
Best,
Ron