American Eagle is Federals econo line. FC stands for Federal Cartridge. I have no idea why they don’t mark the cases with caliber unless they’re using the same cases for 10mm and .40 and then just shortening them for .40 use even though 10mm typically uses a different primer. Could be a cost cutting thing.
One some American Eagle .223 you’ll also see LC (Lake City) brass used with the NATO cross (plus sign in a circle) and the year, no caliber marking. This is because Federal is running Lake City.