There is no way to identify the vintage Marcel. Sorry... On the other If you could please take a closer pic of top and a side view that would be most hopeful. (close on the side too please)
Identifying Coticule layers is a bit like identifying mushrooms.
We could sketch up a sort of taxonomy, but without proper experience, misidentification is very likely, all the more when one can only go by pictures. That said, I'm not one to withheld information, so her goes:
(from: http://coticule.be/the-cafeteria/topic/1727.html)4. La Grosse Blanche.
Pale colored Coticule layer, that turns deep brown when exposed to (sun) light for a prolonged time. Black manganese lines and dots may be present. Sometimes brown needle-spots, spread over part of the surface, resembling pores (although the surface is non-porous).
Slurry has a characteristic soapy texture. Speed on slurry is fast to moderate. Speed on water generally very slow, but the presence of said brown spots predicts a faster behavior on water. Usually much "slurry dulling", but mellow, skin friendly edges when finished on water.
I can't say that I've ever seen a coticule that looks like that. I wouldn't have been 100% sure it's a coticule if I just saw pictures on, say, eBay.
Since it's slate-backed, it almost certainly would have come from Ardennes-Coticule. Maurice would likely know more about it if you sent him pictures.