To answer the original question - Ed is right, they were originally treated with raw linseed oil.
An absolutly mint unused rifle will have a raw matt unfinised surface to the woodwork - but depending on the amount of use and the age of the rifle the surface becomes smoother and filled with subsequent applications of oil and finger grease etc. which can give the appearance of a thin BLOicon oil finish.
A good approximation of an aged finish can be made with some fine steel wool and wiped off applications of BLOicon over the RLO.
A varnished finish (apart from commercial rifles) means someone has tarted it up afterwards.