From Insulator Wiki (Wikilator)
Consolidated Design Numbers
Created by N.R. Woodward in the 1950s, this is a numeric system categorizing all known glass insulator shapes, and assigning a unique number to each one. Pin type (both threaded and threadless) as well as some non-pintype glass insulators are covered. The system was publicized first in his "The Glass Insulator in America 1967 Report" and then more widely by Marion Milholland's guide books.