You mention the 1930's and 40's. Did the people you're wondering about become young adults at that time? Did they belong to the same ethnic or religious groups? If so, that might narrow things down. Perhaps they felt comfortable reject the naming traditions their culture or religion imposed but didn't want to go to the trouble of legally changing their name. Some cultures name their children after grandparents, uncles, and aunts. Others name their children according to the saint's day nearest to their child's birthday.
Some of my ancestors are Scottish and they named their children in a very predictable way until the latter part of the 19th century.