In 1961, my U. of Chicago Physics prof showed us an actual device used by M&M to measure the S of L. As I recall it had two mirrors facing each other (or maybe a light source and a mirror.) One or both of the mirrors was rotating rapidly.
I remember being surprised at how small it was. As I recall, the two mirrors were only separated by a couple of feet. Because of the rapid rotation, the time taken for the light beam to make the round trip to the rotating mirror and back could be measured by a noticeable distance between the source of the light and its reflection back.