F-91W Micro SD Addon
This addon allows you to attach a micro SD card socket to the back of your F-91W for carrying around data. You will need a small cross-head screwdriver to install the addon. Also be aware that you will need to bend the 2 contacts up so they still touch the inside of the metal backplate in order to have the alarm beeping sound still present. Check this video out for more info.
Note: The watches/add ons ship from the UK.