I think that since there isnt ant APA standard here in the United States for the GCM, It is wide open as to what the 'standard' is. I know that Bev's Marans list them as e+ birds, I also know other say ER is proper. I'd say whatever the French Standard for them is probably the one to strive for. But the English do different things with the standards as well. The good part is that we're basically developing a new standard that hopefully gets adopted in the US. If you go by chick down look, e+ chicks look chipmunk, which isn't what most GCMs seem to look. They are dark, sometimes with the lighter belly like most ER chicks.
Now the French marans club says they are traditionally e+ based, but can be based on other varieties, which opens up the ER and E^wh varieties. Which is correct, I don't really know. I know what I prefer though.