I am sure you saw the Robertson Robert Ross had at Keuka when you took delivery of your canoe. Two photos of it are below. The deck configuration is a little different than yours but the overall shape and size of the two have some resemblances.
I did not exactly see the picture I was looking for of the stem/garboards so I will try to explain. I will probably end up talking myself into circles on this one So round and round we go.
As you know the two opposing garboards are butted together along the centerline of the canoe and fastened along the bottom of the stem. As the garboards make their nearly 90 degree twist to the sides of the stem there is a transition in which the garboards leave the bottom of the stem and end up on the stem face. This is usually aided by a rolling bevel.
As you can imagine the profile of the stem can dictate how or when this transition is achieved. A lot of builders either have no rolling bevel, they poodle faked it by running a continuous bevel down the stem that just stopped somewhere around the last fully bent rib (more or less) or they put a quick rather abrupt transition in their bevel that kind of works.
Robertson and some of the other Charles builders seemed almost meticulous about this part of building. In the second picture above that you posted of the stem, you can notice the bevel on the stem is carried pretty far back, almost back or even past where the keel to outer stem joint is, at least that is what I see. This is typical of CR builders. If you can imagine how the garboard would run and transition along this bevel you might be able to envision that the bottom edge of the garboard, or the edge running along the centerline, would have to be finished off somehow.
It’s been my experience that Robertson canoes would leave the plank edge full thickness and plane it off parallel to the bottom of the stem. He also seemed to cut his bevels fairly precisely. Other CR builders would thin the thickness of the plank edge. Does any of this make sense? It’s probably more than you ever wanted to read about stems, garboards, and bevels.
Also the joint between keel and outer stem on your canoe, the simple bevel cut, was, I believe, the way Robertson did it.
Just keep in mind that these are just observations and by no means a definitive answer to the way Robertson did things. I am sure more than one Robertson exists that blatantly defies all of this. Real helpful, eh?