Porcelain, Ceramic, Wood and Composition
are the materials that light sockets for Christmas light strings were made of pre-Bakelite or before the 1930s. The earliest are white porcelain light sockets. These were only in production a year or two. These were followed by green porcelain sockets. Wood sockets were used on battery operated sets.
Composition is an almost clay-like substance used in light sockets, dolls and other toys of the era. It is found almost always in green, and rarely, red. It is usually heat resistant, quite heavy, and is sometimes confused with both wood (lighter) or ceramic/porcelain (shinier) by novice collectors. You can be sure that you have an early (1920s or earlier) light string if the sockets are made of one of these materials.
Information Source: Old Christmas Lights