In your quest for an original type finish, always remember that much of what you will read in various gun forums or books is sheer conjecture. That is why you will get various "expert" opinions that differ wildly at times. In many cases, the factory records and methods have been lost to time, and these companies closed so long ago that there are no surviving employees who know or remember. And in the case of a gun company that was in business for decades, what they used in the 1880's may have been totally different than what they used 40 years later. I have read what supposedly constituted an original finish for Syracuse Lefevers on the Lefever forum, and believe it is total bullshit. But that is hardly the only misinformation to be found on gun collector websites.
As you have seen, the finish on most lower grade vintage shotguns was a drying oil or varnish type finish that was not built-up or intended to fill every pore. Also, old finishes that may have originally been smooth will shrink into the pores over time, and they tend to become darker as they oxidize. Some gun makers apparently used shellac. Sometimes, they used shellac as a cheaper filler base coat, and used something else as a top coat, because shellac is not the ideal surface finish for a field gun. It provides a hard scratch resistant finish, but it will easily spot from exposure to water, and it will chip and crack over time. Manufacturers used what was cheap and available, and as these were production line firearms, they were not treated to meticulous labor intensive hand rubbed multiple coats, and perfectly filled surfaces. BLO or linseed oil may have it's place in some original stock finishes, but it is a very poor choice as a stand-alone gun stock finish, even though some swear by it. Again, many gun makers used it because it was cheap and readily available.
I find it interesting where you say Newell added castor oil to shellac as a plasticizer. I'd presume that was to make it less prone to cracking. I kinda doubt that would do much to make shellac more water resistant.
The easiest way to know if a gun was finished with shellac is to wipe the surface with denatured alcohol. Shellac dissolves very easily in alcohol, and varnish does not. Of course, that would not be a great idea if it removed a good original finish on an original Parker, or any other gun. Several years ago, I was able to completely strip and clean a grungy, chipped, and scratched finish on a 16 gauge L.C. Smith field grade. I wiped on a coat of shellac as a sealer to even things out, and top coated with a varnish.
About that same time frame, I picked up a very nice Baker Batavia Leader 12 gauge at a gun show. The blue and case colors were very strong, but the stocks looked horrendous. They were dark and looked like an alligator hide. I immediately knew that someone had applied a fairly heavy coat of shellac, and I was able to buy it very cheap because of how ugly the wood was. It took very little time and effort to completely remove the shellac that had been applied, and thankfully, the original finish remained, and was still in very nice condition. I saw the very same thing with a Lefever G Grade that I bought on Gunbroker. The stock was as ugly as a mud fence in the photos, so it sold very cheap. I was able to make it very presentable, and preserve the original finish underneath, for about a dollars worth of denatured alcohol, and an hour or so of time. That one even had shellac slopped on the metal and buttplate, and a little alcohol easily removed that as well. Sometimes you get lucky.
I admire your desire to refinish your gun using materials and techniques as near as possible to the original finish. But I am not a Parker collector and only have a few Parker doubles, and an 0 frame stock set. I could well be wrong, but none of them look like shellac finishes to me. And sorry, but I don't have any reason to slop on some alcohol to find out for certain. If you had a definite original Parker stock, you could give a chip of finish to someone in a good crime lab, and they could run it through an NMR spectroscopy test, and perhaps tell you exactly what you have. But even then, they would need something to compare the results to. Another caution is that a helluva lot of Parker shotguns that are said to be all original are not. However, if I was going to refinish one of them, I do not think I would choose shellac as a top coat on any gun I planned to use for hunting, knowing what I know about shellac. Just something to think about, even if not what you wanted to know.
EDIT: I see BrentD reading my post again, when he claims to IGNORE my posts. I wish it worked the other way, so I could block him from reading what I post. I hate possibly providing info to someone who voted for Joe Biden.
