This page contains some pictures of the inside of Sega Master System 1 and 2 consoles, which illustrate where to fit a 50/60Hz switch. Many thanks to H. Meun for creating these pictures. It may be helpful to download these and print them out before starting work on your console. They are in JPEG format.
Some of the files are very large, since they are at 300dpi; you may wish to only download the lower-resolution 150dpi pictures.