Anyone? Joe, Dennis, Jer? Itchin to try this beast.
That is one of the 4 standoffs used to locate the backplane boards, provide a thermal conduction path for the bias transistor and the frame ground connection point to the chassis.
There are 2 for each of the backplane board pair. Look behind the control board and confirm that there are 4 standoffs like this that are in place. If there are, then this is just an extra that got left behind in the assembly process. If not you are missing an element that should be restored to its rightful place. These standoffs are 6-32 on the top 2 locations and 4-40 on the bottom 2. That will help you zero in on where to look.
If the standoff is from one of these locations, then you have a loose screw floating around in the packaging as well and possibly up to 2 internal tooth lock washers as well.