I think she is a badly colored BCM, American style, which means without feathered feet.
Marans eggs do come in all shades, though always brown. What you got was lighter on the scale, maybe a 3 to 4, so not technically dark enough for Marans scale. But alas, even well bred Marans will throw a progeny with a lighter egg. (I paid really good money for a Splash Marans who came from a 7 line but laid about a 4 to 5). I expect she is hatchery quality.
They always lay a bit darker at the beginning of the cycle, and eggs are smaller at the beginning of any newly start lay, even with older hens. First year pullets remain smaller while the second year layers soon ramp up to larger eggs.
Without seeing the rest of your flock, I say you do have a BCM who is producing those eggs rather than an Australorp...she has the classic body type as well as the mohagany bay colored eyes of the Marans...does she have pink/white inbetween her toes? Another classic sign.
LofMc