1972 Demon with 71 340 casting

Hi all, I have a 72 Demon that I have owned since I was 16 years old (for 30 years) that I have recently restored. I was always under the impression that it was the original 340 with the car. As I have become a little more educated on A bodies through this site, have started digging into it more and it appears that may not be the case. The casting on the block reads 1 18 71, which I assume means casted on Jan 18, 1971 (photo attached). Is this definite proof that this is not the original 340 for the car? Is it possible to have this early of 71 motor in a 72 car? Can't say I would be disappointed as I know they dropped the compression / valve size / HP in 72. Thanks for your time.

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