Parker Bros. used shellac (French Polish) on just about all their guns. Customers could request an oil finish if desired.
Shellac is a very poor "varnish". It would not last two weeks under constant handling. I wasn't there, but I feel pretty certain Parker used a modified linseed oil product on their stocks to resist water and handling.
"French Polish" is nothing but the process of applying shellac by rubbing it on in several layers. It is ok for interior furniture or some musical instruments that never are in outdoor use, but for a gunstock, it would be unsuitable.
JR