I guess the slot you can see in the cover is there to poke something inside to determine the combination.

In this case:
Ideally this something is as wide as the slot long is. Push this something slightly in and turn the dial until it slides in further due to the gate being there. The dial won't turn then further. Note the number. Reverse direction and turn until the something slides again in. Note number. Reverse gain, turn to stop, note number. The fourth number you can directly see when the wheel pack is removed. Turn until the gate there is at the fence and note the fourth number.