Lee,
I spend much of every fall and early winter going over photos of hummingbirds, hoping to find a vagrant among them, which I can then drive out to band. I say this just to comment that ID by photo is something that is considerably different from ID in the field. Much can be hidden, or misrepresented in photos. In the photos of the apparent Canvasback, I see a narrow sliver at the base of the bill which appears only to be on the bird's left side, but not even in every photo. So, the photos do not assure me that this is a real marking, rather than just some stray reflection or other photographic effect. Shouldn't it appear in every photo of the bird's left side?
In any case, here along the Detroit River, we get a chance to view 5-10% of the world population of Canvasbacks every fall and winter (my all-time peak count has been 28,000). I confess I don't look at them all, but do look at them a lot. I have never seen anything except a completely black bill on Canvasbacks here. I have not seen a marking such as is apparent in only one of the British photos. I don't know if this helps any, but this is my perspective from someone who sees so many Canvasbacks each year that they're often ignored in pursuit of other species.
Here is a small sample of photos, from my website: http://www.amazilia.net/images/Birds/Waterfowl/Canvasback.htm. Note that a couple photos of females show a very narrow pale crescent of feathering at the base of the bill, which may or may not be analagous to what you're seeing on males...
Allen T. Chartier
amazilia1@comcast.net
Inkster, Michigan, USA