In the picture below, the rain sensor input can be seen. It is the green two wire connector on the right of the device. On the far left are two connectors for the common return wire and the 8 way connector in the center is the 8 zone output block. On the left side of the case you will see the orange power input connector and out of view on the rught side is an 8 pin expansion connector.
The rain sensor may be either normally open or normally closed and this setting is changeable via the software.
On the latest version v1.4 the opening panel in the center of the case is blocked off but is still visible. It severs no purpose on the OSPi.