I have encountered it MANY MANY times in German original stocks, Norwegian regurgitations and even RC stocks. German stocks seem to have used something akin to vaseline or a quite waxy grease as the stock sealer INSIDE the channel etc and to keep out water along the edges. 22 commercial rifle, but NOT on a military rifle. I am happy to use it on my Mauser Mm-410B.
It does work well, but it also adds another layer of something the Germans didn't use, and it does darken laminates. Some people (AHEM! LOL) used to use Howards Feed N Wax, but I see this has gone by the way side, thankfully. I no longer chase any laminate, so will defer the preservation of same to others, as, simply, I am sure my info would be wrong on laminates, though pre-WW2 laminates probably were treated as above (again, they show many indications of such treatment).
So, technically, you COULD "rejuventate" a pre-WW2 stock with linseed (as the pre-war stocks show definite tell tale signs of linseed polymerised finishes), but if the stock does NOT need it, DON'T! I will clean down a dirty original stock with either Ballistol or even Bill Reed's old chestnut of vaseline wipe down on a towel.
This eventually changed, as linseed became less and less available, to Ballistol (in German use for decades prior to WW2). As far as I recall, the Wehrmacht gave orders pre-WW2 for rifle stocks to be maintained with lackierend liensemenol - "lacquering linseed oil" - I believe this roughly equates to boiled linseed oil, or a Danish Oil equivalent (not the modern garbage with polyurethane etc in it, but the real stuff).