I'm gonna guess barred rock. The females have a pretty solid white spot on their head. The boys have a more scattered area of white... But I don't think barred rocks are rare???
Its supposedly a free " rare " breed . Hopefully the hatcheries are not calling the barred rocks rare .
Edt : My bad , that WAS NOT specified by the OP . Both pics make me lean towards a Marans anyway .
2nd Edt : Whoops , its in the title , it IS supposed to be a " rare " breed