From the Jan. 22, 2014, Shorpy "Reflections on a Stop Sign: 1936.
March 1936, no caption on photo, but of an early reflective stop sign.
We take reflective stops signs for granted today, but at one time they were not necessarily reflective.
Comment from rvdroz saying the sign was probably yellow. The octagon shape was adopted fairly early. making it red was optional since there was no durable inexpensive red paint.
The need to reflect headlights at night led to extensive use of small, round "button" reflectors inside the text (as shown in the photo). This sign may have been testing letter coloring and reflectivity.
The Development of Driving in the Early Days. --RoadDog