late 1950s. They took the form of an oversized rectangular postcard with the usual address and greeting space on one side and an illustration on the other. The illustration was overlaid with a transparent plastic material into which the grooving was impressed. The recording was usually musical as the name implies. They typically played at 45 rpm. It was recommended to not write on them with a ball point pen, an invention which was just coming into common use at that time. Laminated cardboard records have been produced as integral promotional novelties on packaging, most notably on the backs of cereal boxes in the late 1960s and early 1970s. Beginning in the 1940s, flexible records began to be used in the form of "book records"–spiral bound paper publications and four or five flexible record sheets bound in. A spindle hole went through the entire assembly. Book records could be opened to one of the records and completely folded back around itself, so that the whole thing could be placed on the turntable and played intact. In the 1950s, advances in vinyl production technology led to the development of the 7-inch 33 rpm flexi disc record. Only seen occasionally in the 1950s, these recordings were increasingly used as inserts in magazines that included audio supplements from the 1960s through the 1980s. The recordings were pressed on very thin, flexible sheets of vinyl (or laminated paper), providing a mixture of economy, practical utility and novelty appeal. Flexi discs or Soundsheets were often provided by music publishers to their customers, frequently school band and orchestra directors, marching band and drum corps leaders and others, with their printed catalogs of sheet music. The director could then hear a sample recording of the piece as they looked at an excerpt from the musical score. In the late 1970s and early 1980s, when computer programs and other binary data were often stored on audio cassettes, a number of microcomputer hobbyist magazines published "flexible program sheets" under various trademarked names including "Floppy ROM", "Flexisoft", and "Discoflex". These bound-in thin plastic 33 rpm audio recordings stored computer data such as video game programs that would be played on a turntable and dubbed onto a cassette. It was also possible to connect the record player's output to the computer's cassette (analog signal) input port and load the data into the computer directly. This method of storing computer data later