WebmasterWorld thread named YPN in 1969 reports YPN publishers getting payments marked as paid in 1969. It's a common issue found with Unix based programming, even Google had an issue with cache dates from 1969.
Besides for that, some members are reporting
I also got paid C, not money, but the letter C in 1969.
I did not get paid for C. It says I got paid for the number 1 in 2000 and 2001 and the number 4 in 2003. So I guess that means that I own two numbers.
I find this funny, reports show that YPN support is aware of the issue and working to resolve it.