120 film is designed to permit a variety of different sizes of negatives. It is 6 cm wide, but various cameras are designed to shoot negatives that are different lengths - 6cm x 4.5cm, 6cm x 6cm and 6cm x 9cm being three popular formats. To accommodate this, there are three sets of numbers printed on most backing paper - each set of numbers is a different distance from the edge of the backing paper. The red lenses are designed to reveal the set of numbers that coincides with the size of negative produced by the camera.
A few cameras allow multiple formats. For example, some cameras have masks and other features that allow you, for each roll, to choose between exposing 8 negatives that are 6cm x 9cm each, or 15 or 16 negatives that are 6cm x 4.5cm each. Those multiple format cameras need separate windows for each format. If your camera only has one window, it was designed to produce just one size of negative.
It is not unusual for there to have been several models of a particular camera made over a period of time, with only small differences in features distinguishing each model. Websites that collect information on the various models of older cameras rarely show pictures of all the different models.