I guess there are no takers.
Ball over diamond over ball means it's a vessel restricted in her ability to maneuver. Which a dredge definitely is, though it's not the only type of vessel operation that fits that criterion.
But the big one is the ball over ball and diamond over diamond, which indicates and is required for underwater construction (including dredging), and they further indicate 'safe to pass side' (diamond over diamond) and 'danger side' or 'obstruction side' (ball over ball). So when you come across a dredge operating in a channel, remember that 'balls are bad' and 'diamonds are delightful'.
Oh, and at night, it would be red over white over red for a vessel restricted in her ability to maneuver, with green over green on the 'safe side' and red over red on the 'danger' or 'obstruction side'.
You can't always just stay away, especially in a river or channel. The river I used to dock in was dredged a few years ago, using a crane with clamshell (no pumping so no hoses/pipes, which may not be obvious anyway), and it wasn't obvious which side to pass on when it was in the middle of the river (quite narrow) at least until they swung the clamshell over the side.