The Hollis is likely restorable if it's been used with modern shells.
Hinge pins wear, but mostly on el-cheapo Belgium guns of the period.
Rust under the ribs sounds bad, but fairly normal. It's from the ribs and not the barrels themselves, better scenario.
There's a thread on damascus barrels (twist steel) on the ALR site right now.
I plug this channel on their site, so??
ALR - www.americanlongrifles.org