I looked at the Library list for decoding dates and the C4 circle data does not appear to apply.
Just guessing but:
2 dots for February, 28 for a day and the rest is unknown other than the 66 (C6) engineering casting number.
Without other data that would be 2/28/66, 2/28/67, all the way up to the next casting number change for the C6 trans.
It would be interesting to have people with tag numbers to display the tag and their circle data. With more data points we might be able to figure out a trend. Then again, how many were changed during a rebuild?