They were? I didn't know that.
The ducks were made by multiple manufacturers and each model is the same i.e. the hollows are double ground, the wedges are single ground.
I've never seen a Hess 44 (don't remember the 42s) that wasn't double ground. Here is a profile shot of a 44 (btw this was honed heavily at the tip so it's less pronounced but that's the one I found a picture of, not of the unhoned ones):
Attachment 159977
It's very easy to notice the belly if you look at the reflection in the blade, a double grind is two concave mirrors and one convex between them while single grind is just a single concave mirror (those are cylindrical of course, there is no curvature along the length of the blade).