If my memory is not failing, (it's getting harded to streach the old gray stuff back that far) there should be 4 screws, one under each rubber pad.........Also ther may be a single screw under the center of the back label, feeling for it with your thumb nail may confirm this.
I would remove one pad to check.
If it has not been charged for a very long time the battery's may have leaked, hopefully not, as this is a good find.