That's a legacy of a Yacht and Boatbuilding apprenticeship and a career in Norfolk boatyards.
Although I have never had cause to use one, a declivity wedge is used with a spirit level to compensate for the slope if any work is done on a boat on a slipway.
"Declivity wedge" has a nice comic ring to it, so is often heard in various contexts on a boatyard, much like the Golden Rivet I suppose. Best not to ask to see the Golden Rivet if you don't know what it is! (They do exist though, I have fitted them.)
Declivity just means slope, or deviation from level.