Yes, I THINK (from what I've read on BYC) that you are correct. White Leghorns OR White Rocks are used with a RIR rooster to get the "Red Sex-Link".
However, She had a previous "hatch", but only ONE hatched. It looks like a Buff Orpington but has some BLACK in the tail.
I'll try to remember...
I have 4 new chicks hatched and raised by their Momma, a Buff Orpington hen.
Their "Daddy" is a Red Sex-Link Rooster.
ONE of the chicks is the color of a Buff Orpington. The other THREE look very much like your WHITE ORPINGTON chicks!
I'm really curious if any (or all) are...