When were the 8 3/4" rears available for the 1965 A-Body cars

Just wondering......if Chrysler knew by spring 1965 that the 7 1/4's were breaking with far too much regularity behind 273 HP engines a 4 speed transmission, why did both of my 66 Barracudas S 4 speed cars come from the factory with 7 1/4" rearends? Especially since factory letters to dealers in early 65 advised replacing busted 7 1/4's warranty rears with 8 3/4 replacements. Go figure.