Dave,
you are doing a spectacular job. This is the most complete and accurate listing that has ever been published.

For a number of years - 40 or so - the factory was mum about the engine sequence numbers, I can speculate this was a method to verify a case was as produced. I used it as a "tell" when looking at the 232 cars from 1964 and as you can see it is very systematic for that period.

You have uncovered one more of the factory intentional or unintentional "secrets". Well done. Thank you for filling in our knowledge base. Keep the posting coming...this is good stuff.