There are a number of different filament mounting styles that have become standard over the years. These are not specified in any national or international standards, but a comprehensive system of nomenclature was introduced by General Electric around 1910. It has been adopted by a majority of other lamp manufacturers and thus exists as a kind of industrial standard.
The system consists of a two-part coding. The first letter identifies the type of filament wire employed (S = Straight Wire, C = Single Coil, CC = Coiled Coil, FC = Flat Coil, R = Ribbon, B = Braided). The second part is a number which indicates the shape of the mounted filament, according to the illustrations shown in Figure I4.
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Figure I4 - Filament Mount Shapes