As I said you got that switch at the front that switches between the manual- and the auto-mode.
Actually it is no electrical switch, only an optical one, in front of the sensor. Typical for a lot of flashes.
One setting is the yellow one, the other the green one. You just have to set the given coloured aperture at your camera, dependant how you set that switch and what speed of film you got. The same for all auto-exposure flashes, except for those that communicate with the camera.
Typical "auto-apertures" are (@ISO100) 2.8 and 5.6. As with yours.