The ES-5 is an expansion module for the Expert Sleepers ES-3 Lightpipe/CV Interface.
It piggybacks on one of the ES-3's stereo channels to provide eight gate/clock/sync/MIDI outputs on the ES-5 itself, plus further expansion headers to connect up to six expansion modules e.g. the ES-4 Gate Expander.
The front panel is drilled for both Doepfer and Analogue Systems mounting holes (using a 'slot' rather than a single round hole). The power connector is a Doepfer standard 16 way IDC, with -12V furthest from the top of the board.
Note that connection to the ES-3 is by a simple plug-in cable for the ES-3 mk2 only. The original ES-3 requires some soldering to add the required connections