Shellac was the original finish on transitional planes and on a lot of other wooden tools in the late 19th and early 20th centuries. It was cheap and fast drying, which makes it much better for factory production lines than any oil or oil-based varnish. It was also used extensively on furniture at this period.
You can tell if an old finish is shellac by dissolving a small area of finish with alcohol on a Q-tip. It also has a distinctive smell which you can bring up by lightly sanding the old finish. I have found it is the usual finish on saw handles from this period as well as transitional planes. I think it was often applied by dipping, as I have occasionally found thickened sagging of the finish, for instance at the bottom of a plane tote, where the excess shellac has beaded when the piece was hung up to dry. Spray technology really came in at the same time as nitrocellulose lacquer in the 1920s - the automobile industry had been clamoring for a better finish than oil-based paints and varnishes, which were a huge bottleneck in their production. Furniture factories took up this technology as well, and if you look at a lot of furniture of the early 20th century the change in finish appearance is pretty obvious. I know that Disston started spraying their saw handles with lacquer at some point - it decays in a different way than shellac, first crazing and becoming semi-opaque, then disintegrating into a powder which comes off on your hands. I haven't seen a transitional plane with lacquer finish, though since these ware made into the 1940s it is possible that some were sprayed with lacquer, if the manufacturers bothered to update their methods.