They used what most every other gunmaker used-a linseed based varnish. They all had their formulas, but it was all just a varnish using linseed oil and perhaps a little tung oil and thinner and maybe some stain mixed in. A good replica would be 1/3 Tru-Oil, 1/3 boiled linseed, and 1/3 mineral spirits, with a dab of Japan dryer.
JR

Be strong, be of good courage.
God bless America, long live the Republic.