Could be called king cornice the 3rd for all it matters. You'll have to search the net looking for a matching profile. Or find the original seller. Or pay for someone to make it as a one off to match. I don't have that profile. Have similar though.
There are so many different cornices , unless he knows who originally made it would be simpler to get it remade .
We have made up five different cornices for an irish builder working in London recently, said he can't be arsed traipsing around looking for it,