I was just browsing through my copy of the DCC300/380 Service Manual and as suspected it is virtually identical to the DCC60 Service Manual apart from a small section from the user manual on the start.
However, mine has some handwritten correction in it for the service mode.
Key-Test: press “MARK WRITE”; display shows “KEY TEST”
The Hole-Test was partly crossed out, so now we have the closest to an official confirmation we will get that DCC300/DCC380, unfortunately, don’t have a Hole-Test.