A56 was a 69 only option I believe for 'Cuda pkg. A53 for 69 was Formula S. You got one, the other or neither. You couldnt mix them to the best of my knowledge.
Cool car! Pics when u can
This applies to 69 only. A57 is the code for ‘cuda 383 with E63 showing for the engine code. A57 and E63 were also coded for the 440 ‘cuda except that the M in the Vin made it the 440.