LilysDad,
I'm a cabinetmaker and have done a fair amount of restoration on early cherry furniture. I don't know specifically what 18th &19th century gunsmiths were doing but in understanding the finishing techniques and practices of cabinetmakers of that era there are a few interesting factors to consider. Linseed oil was already a very commonly used finish by the 1700's and they well understood that the raw product made for a soft and very slow-drying finish. In that era, "boiled linseed oil" was made by boiling the raw oil with lead oxide which made it dry both faster and harder. As you say, these guys were making a living and needed to get their products finished and out the door, so "boiled" linseed oil made sense for cabinetmakers (and likely for gunsmiths as well). What they may not have understood is that the added lead oxide, like lye products, is alkaline, and has the same effect on cherry (though more slowly at a lower concentration than our finish of oven cleaner, baking soda, etc). So your question about them using multiple-component finishes may be that they inadvertently added them all in one convenient step by using their version of slightly-alkaline boiled linseed oil.
Also, different woods respond very differently to natural oxidation, as you say "time and sunlight". If you plane-smooth two boards, one of cherry and one of walnut, over a long span of "time and sunlight" the cherry will continue to get darker in color and the walnut will continue to get lighter (assuming no one's oiling or continuously handling them, oiling will darken them both). I've experimented with 2 chunks of the same unfinished cherry board, one kept in total darkness and one in direct summer sunlight and just the sunlight darkens it a great deal in as little as a day or two.
My guess (FWIW) is that cherry gun stocks were much lighter in color the day they left the maker than what we see today but also that they darkened very quickly with use. No way to know for sure but that's my take on it.
John
PS. Robin, That cherry stock looks great!