From wikipedia:
These Monsanto Application Notes from 1970 are for the LED Museum.Until 1968, visible and infrared LEDs were extremely costly, on the order of US $200 per unit, and so had little practical use. The Monsanto Company was the first organization to mass-produce visible LEDs, using gallium arsenide phosphide in 1968 to produce red LEDs suitable for indicators. Hewlett Packard (HP) introduced LEDs in 1968, initially using GaAsP supplied by Monsanto. The technology proved to have major uses for alphanumeric displays and was integrated into HP's early handheld calculators.
One interesting article caught my eye.
"Monsanto GaAsLite Tips," Vol-1, 1970 (pdf, searchable, ~1MB)"Film Annotation Using The LED" By definition, annotation means to make or furnish critical or explanatory notes or comment. Conventional methods of film annotation such as the neon lamp, spark gap or gas discharge systems left the instrumentation engineer with much criticism and little explanation/information on his film. The light emitting diode LED will solve this problem because of its many advantages over the past methods used in film annotation. The most common use of the LED in photo instrumentation is the annotation of time and/or event marks on the edge of the film. This GaAs Lite Tip will discuss the advantages of the LED and the types of film recommended tor use with the LED.