I picked up a nice Emperor Napoleon, which I think should be Scott #24, Yvert #35, green on pale blue, issued in November, 1871. (The other choice is Sc#23, issued in 1862, yellow-green on greenish.)

Sc#23 has a much lower catalogue value, so I would start out expecting that is what I have, but I think it is #24. The #23's are much more yellow, to my eyes.
Anyone else care to guess?
The cancel is a light strike of a pearled cds from Erquy, a small town on the northern coast of France. The (21) in the bottom of the ring represents the department within France in which the town is located, in this case Côtes-du-Nord, now known as Côtes-d'Armor, in Brittany. The cancel appears to be '1872' which fits in with Sc#24 (though it doesn't rule out Sc#23).