According to Jerry MacNeish's book, they began making the 0010 block in April, 1969.
With regard to the difference between the casting date and the build date, I'll defer to the experts. However, there were definitely exceptions to the rule. I have one DZ block (authenticated by Jerry) where the casting date and build date are different moe than a month. The only explanation we have come up with is that some blocks had to be returned for re-work. (I don't have the block here, so I can't tell you that actual difference.)