LOL.. yes that Margaret and Amigo pairing in both pedigrees.
I have a couple test pairs currently set up that fall below Russian Black... one is an Aby Blue that I am pretty sure will be Dom to the Brit fur.
The other is an Oci Chocolate x Brit carrier (starter cat that hides BSH C&W). So we should eventually have proofs that narrow this even more.
But the above pedigrees at least give us some guides as to where to be targeting a potential placement.
***Updated to add that the Kittycorn (hides BSH C&W) x Oci Chocolate pair dropped a British Shorthair C&W kitten. Unpacked to test. Hidden could be Oci Choc, but there is also a Bali Cream a few generations back in that pedigree, so I am keeping these parents paired for another week or two.