Nice find! I'm jealous that your cabinet has the certificate of authenticity....mine is MIA.
As for volume control, you can see in your picture of the back of the monitor, the grouping of 4 dials to the left of the monitor has one labeled volume control.
Does your speaker hum when no sound is being produced? Mine does, and I noticed on the DK cab I was playing at the Arcade Expo that I attended in Banning, CA, a couple months back. Not sure if this is true for all DK cabs, or if its something that needs fixing.