I've been reading all over the place, but I feel like it was in a 1951 bulletin written by Asmundson and Lerner called "Breeding Chickens for Meat Production". It's in the public domain, so you can find it online. Again my information is dated, but I don't think there is much better available on dual purpose breeds since the appearance of the CornishX. It is possible that breeds/strains have become so degraded since then that early feathering doesn't correspond to growth rate much anymore. However it was something people selected for to improve their birds in the past and I suspect
is worth looking at again. I feel like all advances in chickens I'm interested in was stopped in it's tracks about 50 years ago. Please share any sources you may have found.
Heritage Cornish/Indian Game was a very popular cross in Britain and America prior to the CornishX era. The Indian Game was crossed over many breeds to improve breast quality in the offspring of hens who laid more eggs than the Game. Dorkings and Sussex were both popular hens to breed to in Britain back then. According to my studies this cross gave a nice breast to the offspring... although possibly no better than either parent breed if well bread. Growth rate doesn't seem to have been any better than either parent breed as long as they were all from good meat production lines. The advantages the cross provides were higher rates of fertility, hatch-ability, and survivability to butchering age. Growth rate for all the birds seems to have been fairly slow.
I've been focussing my reading on books from before 1950 and authors who worked with dual purpose breeds, so my information is dated. But in fairness I believe those were the people who knew these breeds best. Another tidbit I've gleaned from my study is that dual purpose birds were more selectively bread for their meat qualities back then, so that while you see a scrawny looking carcass today on a Barred Rock or Wyandotte, many of those birds produced a significantly prettier carcass with a fuller breast than to same breeds do today. That's why I'm so intrigued by Beekissed's White Rocks. They seem to be more like the old birds I've been reading about.
Respectfully, I don't think the chickens were better back then- my thoughts are that the expectations were different than today. Just like a 140 egg layer per year was once considered good, the same might be true for meat birds. I can't seem to find any pictures of the dressed chickens from The chicken of Today competition- might be very enlightening. The 1957 meat bird would have been an improvement. I did find this:
(Sorry I can't seem to find a way to cut the first picture, it's too small to read)