One more data point on this:
In the Jaguar-Daimler Heritage Trust's book, "Jaguar XK-120 In Detail", page 98 shows a period photograph where these jewels are clearly visible. Both the photograph and the detailed caption note that the flat goes foward.
I don't have the book handy, but would be happy to quote the caption when I do, if anyone is still interested.
FWIW. I don't expect everyone to agree (the logic of having the flat facing rear makes sense to me, too....), so "it is what it is...."
Thanks.