I just won a mid-1880s Charles Lancaster double rifle at auction. The pictures of the top rib show what a scope mount that appears to have a single mount on the gun. Any idea what type of scope mounting system this is?

The auction pics can be viewed here: https://www.invaluable.com/auction-lot/a...Q3vMvblda7Ystc_

And while I haven't yet mounted a red dot on a DR, this almost screams to be used for that purpose, with a custom adaptor plate. Something small like a Trijicon RMR or SRO. If for no other reason than seeing if my old eyes and iron sights can match a red dot.