Hard to say. There were lots of runs of the 11X, and there have been at least two runs of 12X. They sort of vary from run to run. Generally, the domey stiff TeeBirds are really overstable. The flat opaque TeeBirds are less stable. I've seen both types with both stamps. From what I've seen the more overstable ones are more likely to be an 11X, but you really have to keep your eyes open.
As I recall the 10X were KC Pros, not Champ. Some of the 11X were KC Pros as well, although the plastic was CE/Proline/Champ-ish. The KC's varied as well. Some are very overstable, some are beat in DX stable. Again, you have to watch the dome.
So, you can't really tell from the stamp. If you want an overstable TeeBird, domey is the ticket. If you want one that is less stable out of the box, flat is where it's at.