My first thought regarding this would to leave well alone! But that is no help to any one, so I have given it a lot of thought since I read your post. Now to begin with I have never come across this type of stock enhancement and hope I never have the misfortune. though what I will say is I have repaired veneered pieces where the veneer is as thick as a cigarette paper and some wood photograph finishes. So my first piece of advice is no abrasives only the skin of your hand, hand skin is equivalent to what cabinet makers on this side of the pond call 'Crocus Paper' with the next curse grade up called "flour Paper'. Second piece of advice is polyurethane is the kiss of death for gun stocks because it is brittle with no built in flexibility for the wood's movement, it also traps in any water or water vapour that finds its way in the wood. So I would recommend using a restoration type of liquid grain filler ('Restoration' meaning it will not make a permanent change can be reversed leaving the item in the same state it was originally).
A good rag application filler that works well is made from the following dark Bees wax, a few drops of Garnet French polish failing that a good pinch of Garnet Shellac flakes, Methylated spirits. The filler is Hydrated Magnesium Silicate (Talcum Powder) completely non abrasive, to adjust for colour I use Vandyke Brown Artist's oil colour. The mix is sort of experiment yourself but for a start two tablespoon's of Methylated Spirits dissolve a pea size piece of Bees wax in it add ten drops of Garnet French polish then add the Talc to form a Tooth paste type of consistency. Finally I add the colouring. Method is simple, place a little of the filler on a rag and work across the grain spreading the filler then use tour hand to smooth it into the grain. If you apply too much Methylated spirits will remove the excess, leave over night to dry then re apply until the grain is filled finally leave for a week and apply a wax polish for the final finish. The filler can be removed very quickly using Methylated Spirits also the filler hardens with age.
