This pops up from time to time, what F13 do I have? Close ratio or wideband?
Well it's stamped on the outside of the box, the box bellow is marked C 374 so this tells me the box is a Close Ratio gear set with a 3.74 ratio diff, other box's may be marked W 418 which would mean wide gear set with 4.18 diff,

Hope this helps.
The stamp is where I'm pointing my finger, there are a few numbers but you only need the last 4