Then Norm I would suspect the carrier shifted and wedge that homemade key and the trip with the carrier unbolted jarred it loose. Even regular keyed flywheels can get stuck when the keys get sheared. I have a few that have been a real pain to remove because of this.
In the case of the carrier I suspect they used a D shape ID of mounting hole and it sounds like some time in past the area got sheared and someone weld on a standard hex shaped nut and ground out the threading.
Of course we all find things like this when DIYers are involved in repairs.
