Urushi stock finish
Eary rifles and many throughout the war used the urushi finish mentioned above; it is easy to identify being hard, shiney, and 'thick'. It is closest to a form of lacquer and when cured is resistant to most all solvents and to water.
It is not shellac; shellac is easily attacked by alcohol and water causes almost instant discoloration; shellac is not a good stock or furniture finish. Though it will 'look' something like urushi when applied.
Late war rifles seem to have little to a very thin finish. Looks like a very weak application of stain and possibly a raw oil. like linseed or tung. This conclusion on my part is based solely on observation and years of finishing and refinishing gun stocks.
It would make sense that the tedious collection of the urushi sap, and its subsequent preparation into a finish was disrupted by the war. It also requires special application and curing conditions that take many hours. A different finish would have required much less time.
If you will have a look at the Banzai site, there are two long articles by Stan Zielinski on urushi.