Using Encoder Pair for Tamiya Twin Motor Gearbox

I have a question about using the above encoders (Product code : RB-Rbo-122). The product page says that it can only be used in Configuration C. The gearbox, however, seems to have changed over time. The link from the Encoder page to the Double Gearbox page shows a gearbox that only claims to have 2 speeds (hence, no Configuration C). I have an old, 4 speed gearbox that I purchased several years ago that I would like to use these encodes with. It is set up in what its instructions call Configuration C (114.7:1), but, when I look at the video for installing the encoders, it does not exactly match the construction of my gearbox. Can anyone tell me why the encoders would only work in a particular configuration of the gearbox, or if they will work with my older gearbox that does not look quite like the gearbox in the video?

The Tamiya twin motor gearbox is a two speed gearbox which can be configured in three different ways (two of which offer the same gear ratio). The encoders were designed to fit configuration C (does not fit A or B). Can you send an image of the gearbox you have?