I believe that loose screw/bolt is for the timing chain tensioner arm, which could be slightly longer than the one for the oil jet. You can reuse the bolt/screw for the oil jet, but some of the other bolts which have threadlock on them should be replaced (they are supplied in the kit).
Tip: When refitting new timing /balance chain kits ensure all locating threads are dry (oil free) to allow the correct torque setting to be applied to the bolts/screws.
The GM part numbers for the two bolts/screws you mention in your first post above are:
Bolt,Timing Chain Tensioner Arm to Cylinder Block - 11518064
Screw,HEX.HD.,M6 x 30,Jet,Assy - 11516425