Ok, I've been convinced to set aside my adult beverage, go to the garage to investigate. I have A65s from 67~72 as well as many frames & crankcases. I do have a complete '69 whose provenance I know. That one is A65TCnnnnn. It uses the old stamps as do other '69 crankcases.

The old stamps used a larger 8 as a rule, so the OP's #s may be 100% legitimate.

The A65x/MYnnnnnn scheme was introduced in 1970 as far as I can tell. All '69s are A65xCnnnnn.
Of course this brings us to '70 where it may be A65x/MYnnnnn or a '67 serial # with -Y.

Then in '71 some where MYnnnnnA65x OR A65x/MYnnnnn.

I think that I'll go back to my AB before I think about this more.