Which devices was this docking station used for?

I've looked a bit further and I think the dock that you have is for an early iPhone.

My reference to notches refers to the Universal Dock, which looks like the one below:

It has a wide tray, into which the appropriate sized adaptor was inserted, dependant upon which iPod was to be used on the dock. The picture below shows a dock base with three different sized adaptors, each for a different model of iPod:

Notches on the underside of the adaptor clipped the adaptor onto the base.

The dock base that you have is for a specific model of iPod or more likely, iPhone.

The connector is known as the 30-pin Dock Connector and iPods such as the iPod Classic could sit directly onto it.

Many Dock Connector bases had notches in them, designed to accept adaptors that would them allow smaller iPods with the Dock Connector to sit on the base. However, I don't see any clip notches in that base and the shape of the end of the side ends suggests that it was for one specific iPod range (perhaps early iPhones), rather than being a Universal Dock Connector.

The grills either side of the connector suggest speakers behind the grill, but without seeing any other connection ports, it's difficult to say how else the base was intended to work.

The connectors tell me that it was intended as a quick-dock device:

1. 1. the original device cable is plugged into the 30-pin connector on the back of the dock, while the USB end of that cable is plugged into the computer
   2. a jack plug to jack plug audio cable is plugged into the LINE OUT port, while the other end (also a jack plug) is plugged into the Line In of an audio amplifier
   3. now all that needs to happen is to sit the device onto the dock base in order to be connected to either the computer (iTunes) and therefore a power supply as well, and an amplifier

As I mentioned previously, most dock bases had notches in them, intended to secure adaptors, clipped into place, that would then have the appropriate size fitting for other models of iPod or iPhone. The base that you have seems to be for a specific device, shaped in such a way that it fits snuggly into that base. Perhaps a second generation iPod Touch, or an early iPhone.