It the trip meter reset knob. Looks to me like your speedometer and your gauge bezel don't match, the speedometer doesn't appear to have a trip meter but the bezel has the spot for the reset knob.
I also didn't swap in the whole dash frame, so I shortened up the bezel and filled it in using a section of the glove box trim. The resto for the dash is in my build thread, there's a link in my signature. The dash resto is in the first couple pages.
