Realized I didn't have a side view with the possible round bullet scrape. So here it is :)

Colonists and people around the settler time were known to shoot rare large sea turtles in the head to stun and hunt them for trophies. If the results do come back that this was the case for this one, the skull would be dated to being fired at around the settlers time, or well over 50 years at least.
And this turtle sure was HUGE