Does it fix the terrible backlight on the F-91W though..
If it's the same movement as the ABL-100WE, I think it may.
The ABL-100WE has an orange light that stays on for either 1.5 or 3 seconds (configurable) and provides some decent light.
The display contrast doesn't seem to be quite as good as it used to be with older movements, though.
The old Illuminator model has a much better backlight. https://www.amazon.com/Casio-F105W-1A-Illuminator-Sport-Watc...