Is that a sheet of castor oil that is spreading down across the side up forward? I am still learning about this stuff but it doesn't seem like a logical place for oil to leak from.
Taking into consideration the funny things color can do in old photos, it seems the same color as the fuselage, and, considering that this is a Frenchman, perhaps it's his visual testament to the "legs" of a full bodied Syrrah.
A votre Santé