When did missionaries begin using standard lessons for teaching the gospel to investigators?
Here's answer to week's question
A uniform "Systematic Program for Teaching the Gospel" was published by the Church for missionary use in 1952, inaugurating "the use of a standard plan of missionary work throughout the Church." The specific format of the lessons has been modified from time to time.
Source: Deseret News 1995-96 Church Almanac, p. 383.