Me thinks you have left out the bats blood eye of Newt, leg of Toad, all mixed on a waxing moon, and may be some other items suggested by those three not so good looking ladies from the Scottish Play.
There are as many gun stock finishing mixtures as there where Victorian gun stock finishers, and the problem is not what the magic potion contained but how it was applied to the wood.
The linseed oil you describe is artists Linseed Stand Oil expensive and slow drying also it would be far better to use the Venice Turpentine used as a treatment for Horses Hoofs you will a lot for your cash buying it that way and it is low cost.
In a nut shell stock finishes consists simply of a form of Lac (Shellac) Gum or Resin, Carrier or solvent, and heavy metal siccative to speed up drying if you are using a slow or non self drying oil as a carrier, then you can add to this grain fillers plaster of Paris finely ground glass and colouring Alkanet is just one of a long line possibly ending with Dragons Blood for the weird and wonderful.
The stock finishes of past times only had the standard artists materials to choose from and the effects of each is well known, also there are numerous books on art Varnishes with some formulations going back hundreds of years. Then there musical instrument Varnishes using various types of Colophony (tree resins) which Venice turpentine and Copal is just one of many going up to the daddy of them all Amber after processing was added to Poppy Linseed or Tung oils and used as a top quality wood finish.
Butter of Antimony darkens woods in the same way as Permanganate of potash does in the end it is down to All Muck and Magic and a lot of experimentation, and from the substances I have mentioned you will see they are all natural but now we have man made resins not to mention the vast array of synthetic dyes and colours we can use today, to end with there are the waxes some natural and others man made also used as a constituent of many finishes and as a rule of thumb the harder the wax the higher the lustre. Erland you have just started on a long road of exploration good luck with your quest sadly I gave up the quest for the ultimate gunstock finish because there are too many to choose from.
