Could anyone here help me with some information or expertise on the correct varnish for 1910's-1940's shotguns?

It does not seem to get covered in the shotgun forums much even though shotguns are about the only US military weapons that should occasionally actually have a varnished stock.

All I can seem to find locally are modern polyurethane based finishes and I haven't had much luck searching the vintage furniture sphere for information either.
Should it just be an oil based varnish?
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