Look at the vent (stand close to it) and it will zoom in. And give you a better look at the bolt.
Or ... just trial and error if you want to go the long way! :D
I have 5 biig bolts and one small bolt so which one is it?
If you stand close to the vent and do the "eye" icon on it - it pops out a picture. You should be able to tell by looking at that and then looking at the bolts as to which one is needed.
