Mannel also states The block at the beginning of 1966 production was the same 1965 C5AE-6015-E casting but with more current date codes. It would continue to be used throughout 1966 and into 1967 production. These blocks were all from the Cleveland foundry. When the Windsor, Ontario plant began 289 production, it used its own foundry to produce cylinder blocks which carried a casting number on f C6AE-6015-C (pg 6-7)
And some More.
The Cleveland block carrying a casting number of C5AE-6015 and the Windsor block with C6AE-6015-C were unchanged except for updated date codes. (Pg.7-4)