The character sheets that come with the core set.
What is the box next to the attribute diamond for? I can see some vague reference to fortune but I can't find where it says what (or more importantly when and why) a value goes in there
The box is for Fortune Dice, as "vaguely" referenced.
If a PC buys fortune die advances, the PC can put the number in that box to remind him how many additional dice to roll when any skill related to the attribute, or the attribute by itself, is asked for in play.
It starts at 0 for all races/nations of man in the current rule sets.