It seems to be highly refined linseed oil+ a staining agent (I don't have the recipe). Look on the Lee Enfield forum, and you will find plenty of info on the original linseed oil
based treatment. If I remember correctly, this involved dunking the stocks in baths of hot linseed oil - not something that is practival for the home user.
It will do the job. IMHO it is both equivalent to (i.e. has the same effect) and will look like the original finish. I would guess that something similar is available from companies like Brownells.
BTW, the color on originals varies greatly, from blond beech to dark walnut, so don't start thinking in terms of color charts and Pantone numbers!
And don't forget, once you have put on the basic layer, go very, very gently with the oil. It is tempting to cut short the waiting time between applications, but too much oil + too fast = a sticky mess. Linseed oil basically hardens off by exposure to air, so it takes time.