This is a replacement battery cover plate for a PocketWizard FlexTT5 or FlexTT6. To replace the plate, follow the instructions in the video below. This will also allow you to remove the battery door to check it for any corrosion. It is a good idea to clean it with a pencil eraser while you have the part out.
Note: Be sure to just pop off the top left corner of the battery cover. Sometimes it can be very stiff the first time you take it off. Using your finger or a small screwdriver is usually best.