0.9mm hex key allen wrench.  This is a small and hard-to-find hex key allen wrench that is required to adjust the set screw on the Omni 60 lenses.  The lenses come pre-focused and firmly tightened, but in the event that adjusting the focus becomes necessary, a 0.9mm hex key allen wrench is necessary to loosen and re-tighten the set screws.  Instructions on the adjustment and focusing process can be found here.