Alternator or VR issue?
It looks like that fusible link is the one to the upper contact?
If so, then something downstream of the regulator is drawing too much current.
Normal flow through regulator to rotor.
Short to Ground in the Field Circuit
If the output wire contacts ground, then there is no resistance and current flow becomes excessive.
Then the fusible link melts.
Remember the normal position for the arm is contacting the upper point.
Only when voltage is above 13+ Volts does the electromagnet have enough power to pull the arm down.
It looks like that fusible link is the one to the upper contact?
If so, then something downstream of the regulator is drawing too much current.
Normal flow through regulator to rotor.
Short to Ground in the Field Circuit
If the output wire contacts ground, then there is no resistance and current flow becomes excessive.
Then the fusible link melts.
Remember the normal position for the arm is contacting the upper point.
Only when voltage is above 13+ Volts does the electromagnet have enough power to pull the arm down.