Since this thread is titled "Rare Chicken Breeds" It is very apparent to me after searching the internet and making tons of phone calls,the true Large Fowl White Cornish should be on the almost extinct list. I've located and spoken to just about everyone that is listed on the internet as well as other leads given to me on the phone. Three of these breeders,one in Va,one in Kansas and the other in Cali all have birds from the same breeder lines going back to 80 yeears ago. There is little,if any genetic diversity within this one color or cornish.
Which brings up another point. If companies like Tyson,who supposedly bought out the breeding rights of the now famed cornish x-bred, Large Fowl white cornsih WAS used in that geetic package. Perdue is another marketer of cornish cross broilers. But wouldn't these companies have white cornish birds still pure to use in getting their cornish x-breds? I just put in a call to the head feed nutritionalist of Hubbard Feeds to see if he knows. I'm next going to place a call the Nutrena's feed guru. Somewhere,someone has got to be sitting on this color and breed.
I think there are a few more around than you have found. Not to mention that they are dominant white and could be used on a Dark Cornish to make a new flock. They are not common, but they are not at deaths door either.
You bring up a good point though. A lot (not all) of the breeds mentioned here are not rare in their country of origin, and many of the breeds we already have are becoming more and more rare. The perception of rare is interesting to say the least. I guess rare means new to us.
It is a bit ironic that newcomers that are interested in the production of fowl meat, are not interested in the Cornish.
The Cornish X or what we call Cornish X are not a simple cross of Cornish X Rock. They are the result of specialized meat strains that have Cornish in their background. The Broiler industry started with Cornish x NH, and the Cornish x Rock. The name and perception stuck. Cornish is in these strains, but it isn't as we often describe it.