I had written "gaskets" not "gasket", because there are two on my DS 1, one on each side of the CB trunk.
After digging around on the site for a while, I've come up with this link that describes the factory standard setup for a DS1 (thanks to Bob Hunkins).
http://www.bobhunkins.com/surprise/archive/000487.html
In contrast to that setup, my CB has a 3/4" bolt coming through the Port side of the trunk. It's the gasket for that bolt that is leaking. I can only surmise that this represents a modification by a previous owner.
As designed, the CB is supported only on the SB side. When loaded, it gets wedged inside the trunk, that's why a single sided support is possible.
I think the person who made the modification drilled, and probably tapped the CB spindle from the reverse side to add additional support.
OK, will see whether I can undo that bolt to get to the gasket. If it's corroded in place, with a 3/4 bolt where the threads are not accessible to penetrating oil, it could be challenging.