When I began collecting pre-1930 superheterodynes, I had a lot of trouble finding information about them and their schematics. The typical resources such as Riders were of little use. Just identifying the manufacturer of a receiver was a serious challenge.
     I learned, from a court decision in 1925, only RCA could manufacture complete superhets due to legal injunctions against any other manufacturer wanting to build them. A few tried to skirt these laws, but the vast majority of superhets were built from scratch or from kits with additional parts.

     Only a few superhets found today will have "brand names" on the front panel and those names can be very misleading. Due to a loophole in the law, local radio shops in the 1920s could legally build these kits. Some would engrave their company name on the panels before they were sold into the community. Holtzmark, Barry, Smith-Pagel, Lavoy, Sampson, and Gilbert are examples. Because these were custom-built, the true identity of the manufacturer of the kits would not be the same as the name on the front panel. A collector has to look deeper.
     To find the real manufacturer, you'll need to examine inside the set. An oscillator coil, three or four intermediate frequency transformers, and a filter transformer are the basis of the superhet circuit. The "IFs" and filter are usually grouped in a row. These specific parts came in a wide variety of sizes and shapes. Once you begin to recognize the different shapes, you'll immediately know the origin of a radio. Some IFs will have the manufacturer's brand name or identifying marks. From there, you can begin to expand your search for information.