Do you know where they source their chicks from? What hatchery do they use? If they are from a hatchery, then they should be black sexlink pullets. If they get their chicks from local farms, well then, they're anybodies guess...
Then they are probably black sexlink pullets. At 8 to 12 weeks old, they go through juvenile molts, and those tail feathers can look a bit strange. Black sexlink cockerels look just like barred Rock pullets at 8 weeks old. They only have one copy of the barring gene, so it's the darker barring...