Engine identification

What's up guys ? I have a 340 I just pulled out of my dart today,the numbers on the driver's side are 2780930-340-4 which from what I've found say its a 340 from 68-73 . On the passenger side I have 5-28-80 how is that even possible? That would mean it was cast in May of 1980. It has X heads on it also.If anyone can clear this up for me that would be great! Thanks .