As technology progresses and the stylus and tone arm became more efficient, the discs become lighter in weight. After 1968, most of the major labels were no longer issuing new LPs in mono. Sometimes albums advertised there can give you a better idea of the general date of the item you have in hand. Indications of phonogram dates are standard by 1976. 1933 - Articles begin to appear in the library literature on how to organize and catalog collections of sound recordings.
Record stores convert to self-service bins.
1952 - Library of Congress begins systematic cataloging of their sound recording collections. Columbia Record Club becomes the first mail-order outfit for sound recordings. After much speculation, stereo. The 10-inch LP bows out of the picture. 78 rpm records are all but gone from the sales floors.