This was the sheared stud. It had obviously been broken for a while as evidenced by the mushrooming of the stud. Broken mounts are not all that uncommon in Maine as we have lots of lobster traps to snag a motor and stall it by stopping the prop.
Sometimes the shaft bends, sometimes the prop, sometimes the gear box fails and sometimes a mount fails. I can't say what caused this mount to shear but I can say having only two mounts left, when one fails, is a little scary.