Yes, that is something to ponder over. Another one - hard to tell for sure, but that looks like the "Vincent HRD Co Ltd" Mercury Crest. I would have expected that bike to have the "Vincent Engineers" type.
I wonder how many photos are in the whole set from that day. The bike was switched from one side against the tree to the other side. Is there a side on view that shows the exhaust pipe side?
I have a photo of the exhaust side, but was asked not to share it. I'll see if I can use photoshop to expunge the image in question and replace it with a tree branch or a big fig leaf.