The hole on the right would be a much stronger point of attachment. Can you see any wear marks on the holes that would indicate where the spring attaches to? You're belt looks shot also.
It definitely does not attach to that cotter pin. On the twin blade 42" on my Craftsman/AYP machine, that spring mounts to a post towards the back of the deck. Sort of in the same location as the oval slot closest to the cotter pin on yours.