I know pretty much nothing about painting, but as far as what was used back in the day, there are other areas in which you could check, such as steam engines, early machine tools, etc. On one of the nickel plating jobs I had done, the plating shop did say there was an older type (color wise?) of nickel plating which they could do. Acid etching is something I have been looking into and may try it out this summer with Matt Lamborn, a safe restoration painter. I don't remember the gasket material as being like leather unless it was soaked in something first. Hard to the touch, but not so hard I couldn't dig my fingernail into it. On the varnish, spar varnish would be a possibility.