This blog is specifically about incandescent lights. Newer LED lights will also have other letters on the prefix side as well. Older lights did not differentiate between the cell type in the serial. This numbering carried over in 1992 and did not reset when the letter serials came out. The non-letter light on top is 30051558 so it is the 51,558th 3D cell light produced. So, the earliest 3 cell light would be 30000001. All lights started at number 1 and worked their way up. The rest of the digits specify the number of the light in the production sequence. The leading digit, 3, specifies the cell size in the non-letter and the leading D and 3 in the letter serial specifies the cell type and number of cells. Shown in the above pic is a 3D letter serial (bottom) from around 2000 and a non-letter serial 3D from around 1980 (top).
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