Polyester resin is generally man-made, but the "resin" referred to in a 1920s AAP was almost certainly a natural plant product. The 1939 book, "Chemicals of Commerce," devotes a whole chapter to Natural Gums, Resins, and Balsams. (Another chapter is devoted to "Synthetic Resins," although ester had not been commercially polymerized at that time so polyester resin is not discussed.) All of the natural resins are exudations or excretions, generally from plants -- think pine pitch (shellac is an exception, excreted by an insect). The term "resin" is used both as a general term comprising resins, gums, and balsams and also as the more specific sub-class of non-water-soluble "resins" such as the "resins" used to make varnish. At this sub-class level, gums are distinguished by by their solubility in water, and balsams by the fact that they contain essential oils. In later practice, gums were distinguished somewhat more from other "resins," and "gum resins" are described as mixtures of gum and resin. See Van Nostrand's Scientific Encyclopedia, Third Edition (1958).Sorry it's resin not rosin. Two very different substances. Resin is man made; rosin is a natural product.
So, the 1920s AAP article *could* have been referring to one of the gums (and probably was, if the instructions call for softening or dissolving the "resin" in water), or it could have been referring to a member of the non-water-soluble class of "resins" (in which case, the instructions would either involve dissolving it in a non-water solvent or possibly in water with something to aid its digestion, perhaps a strong alkali).
Rosin is derived from pine resin by removing the essential oil, turpentine.