"Cars To Have Letter Signs"(PDF). Two Bells. Vol. 1, no. 48. Los Angeles Railway. May 2, 1921. p. 1. Retrieved February 16, 2022.
metroprimaryresources.info
"May 1: This Date in Los Angeles Transportation History". Metro Dorothy Peyton Gray Transportation Library and Archive. Los Angeles County Metropolitan Transportation Authority. Retrieved February 16, 2022. 1921: Large letter signs indicating the routes of different lines are placed on top of Los Angeles Railway streetcars.