Our tours (Pilgrim Office voluntarios) are usually in Spanish. Last summer, I finally got to ask a question that had flummoxed me on prior tours.
On the ridge of the roof, to the east, above the central nave and main altar area of the Cathedral, there is a single bell suspended from a small stone and wooden frame. It has a cord leading from it, into the roof.
Clearly, it was not intended to be a main Cathedral bell. Santiago, and the other really BIG bells in the South bell tower have that need well covered.
Note: the North bell tower does not have bells, it contains the "clacker" the large wooden wheel that makes a pronounced clacking sound. During Semana Santa (Holy Week) it is used in place of bells during the celebration of the Passion of our Lord, when using bells is considered inappropriate.
Back to the small bell. It is about 50 cm in diameter at the bottom and looks more like a chapel or schoolhouse bell. I did ask what it was for. Here is the docent's explanation...
"Back in the day (my term), the official bell ringer and his family lived on the roof of the Cathedral. The cord on the small bell led, and still leads to the area behind the main altar. When the celebrant in the Cathedral below, desired bells to be rung, an assistance would pull the bell cord, alerting the official bell ringer on the roof to ring the main Cathedral bells."
Go figure...