In the photo with the two sitting on each other, the bottom one is a Rev 2. The top one is Rev 1.

The Rev 1 is the one with the volume control knob showing. Sound comes by connecting the speakers directly to the mobo through the connector on the right front of the mobo (the second connector to the right of the volume knob).

The Rev 2 board has no know, and sound is sent through the Jamma harness to the speakers.