I don't think autosexing unless the Marans was a cuckoo. JMO
I think Black Copper Marans (any black Rooster) over a CCL (barred hen) will produce a sex link. Girls all black and boys will have a white spot. At least that's what I've seen in the past. Doesn't work when the Roo is a CCL and the hen is a BCM.
I'm not sure about the Welsummer since they are somewhat autosexing to start with. I'll let you know when my Wesummers crosses hatch out. I'm very lucky to have a roo that was born almost totally grey and had a white spot the size of a dime on his head and no eyeliner. All his offspring with other CCL have been easily sexed at day one so I'm hoping the trait will carry over with the Welsummer mixes