Setting aside the digressions from the original topic I'll make two points.
1. the OP asked about value. Depending on the level of pitting and where it is, I'd start somewhere around $400-$500 as "value" and $300-$400 as an initial offer (if I was in the market to buy, which I am not). The Costos are, as mentioned above, solid workaday guns. It may be that polishing the bores (and opening the chokes to accommodate plastic wads) turns this one into a real peach of a hunting gun. Taken care of it will likely outlive us all.
2. Last time I was there, the NRA Museum had an exhibit on the guns of America's wars. In the Vietnam exhibit they had a Darne, IIRC cut down on both ends. This came with an explanatory card saying the locals liked the Darne-type action because its strength allowed it to close and function, even on paper shells swollen by/in tropical humidity. I'll leave it to the other participants here to hash out the accuracy of the curator's statement.
