General answer: about every sort of transparent film-forming goo concocted by man.
However, when the nitro-cellulose, the goes-bang-WW1-stuff was discovered to be re-soluble and better yet, applied by mass production spraying to lay on a finsih, then everyone had a good cheap alternative to the old, more labor intensive manual finish applications. Even, better, it was readily tint-able to provide an even tone on cheap woods.
Prior to this event, shellac was certainly the most applicable and quickest way to get a decent finish on wood. Remember, today's finishers and public, save a very small minority, have little knowledge or understanding, of what was once a very widely used and understood product. Shellac was and still is a very good and durable finish when used in the right applications. However, that requires either an education or well-informed craft tradition. It does not lend itself to the mass-market, tho' it's pretty easy to teach someone how to get a simple brushed, steel-wooled, and waxed finish that will receive admiring comments.
However, the turn of the century wood-finish market had many products and branches, especially once modern chemistry began to take raw materials and processes away from the almost alchemical magic beliefs and procedures that had evolved over the centuries. As a result in the 19th Century, oil and resin based varnishes started to be 'industrialized. It's not that they hadn't been mass-produced before then, as witness the sailing ships spars, but that now there were quantifiable and testable means of improving the product. Good traditional turps/oil/resin varnish is pretty usable. And today's modern varnishes are just dandy.
To answer the general question put above, the prior posters all supplied answers that are accurate to some degree of another. If an oily, slathering, brushing, rubbing, film-forming stuff could be put on wood and cured dry, ya betcha it's been used.
It is true that the sprayed modern-nitro-lacquer from the early 20th century thru the 1950's or so, is flaky-prone. At the same time Sears was retailing such guns, oil varnish also seemed to be the preferred finish for mass-produced better goods. However, just about any of those variations can be worn to the wood, water-damaged and otherwise age. When new and tougher films were developed we saw them applied to guns, for better or for worse.
That's all pretty obvious and common knowledge. I cite them only because mass produced gun finishes are are of many compositions. Simple testing can be done with acetone or denatured alcohol or mineral spirits on a Q-tip, discreetly. However, as witness by the many volunteered procedures and materials on this board, there is no need for anyone to go far or exotic to get a good finish. Just about any off the shelf product will give you service, chosen according to the duty needed and aesthetics desired.
Read, follow directions,and don't get too excited. As George Frank "Adventures in Wood Finishing" said. "If you don't like it, wash it off and try again until you do like it." It may take a few years and lots of trials, but eventually you'll find out how to apply what makes you happy.
There is, of course, a lot of 'trade' and 'how to' material already on this site in prior strings. Some good info in them, especially from the brethren in the English guild, where craft traditions were preserved. As weel, the local Woodcraft Stores has several books on finishing, that are based on chemistry and application rather than musty hearsay.
There really needs to be a book done on gun finishes and their history using modern museum conservation analysis. However, such work is pretty costly per sample and for the average reader the volume might be prescribed as a night-time sedative warranted to cure sleeplessness.
