Open-backed banjos are typically used for the Appalachian, claw-hammer style and have a softer tone than their resonator banjo counterparts. In order to identify every used, antique open-backed banjo, you would need to be a walking compendium of instrument and luthier knowledge. However, there are some tips and tricks to refining your search, some of which may yield the exact year and manufacture of your open-backed banjo design.
Inspect the headstock of the open-backed banjo for initials, names, logos or manufacture dates. Some of the typical manufacturers include Slingerland, Lyon and Healy, Star, Maybell, Groeschl, Stromberg, J.B. Schall and Rettberg and Lange. Star banjos may have the appearance of a star that is stamped on the back of the headstock, although the stamped image may be wearing away. Older J.B. Schall banjos featured the fifth-string, which was placed on the middle of the neck. Buckabee banjos can be identified by their flowery, sloping peg head (pictured at BillsBanjos). If you find a distinct bump on either side of the top of the peg head and an ornate leaf and flowers inlay, then you may have a Lyon and Healy model.
Look at the tailpiece and the rest of the body, which may also have initials or years carved into the metal or components. If you see a heart-shaped component placed around the edges of your banjo's body, then you may have acquired a Washburn banjo from the turn of the 19th century. The Rettberg and Lange early 20th century banjos used long, thin heel footprints and small, cane-like metal tailpieces.
Take measurements of rims and inspect the wood components for identification of the type of wood used in the manufacture. Spruce is generally a lightly-toned wood while mahogany is darker and can have a reddish appearance. A 10-inch hollow rim with a mahogany neck is most likely made between the World Wars by Leonard and Coulson. If you find an 11-inch rim with 36 brackets and brass and shell inlays, then you may have a celebratory Columbus model made by Dobson during the turn of the 20th century. Also look for the remains of an artist sketching of a ship on the headstock and the dates 1492 and 1892 on the neck.
Take your banjo to an antiques specialist or a specialist in antique banjos. There are a variety of sites, including Mugwumps and Bills Banjos that specialise in identifying no-name banjos and their named counterparts from the late 19th and early 20th centuries.
According to BillsBanjos, many manufacturers in the late 19th and early 20th centuries made unmarked banjos for other distributors and companies to sell.