I would be careful with a cast aluminum stem like that. Is the part that goes in the frame straight ? It looks a little curved in one of the pictures, although that could just be distortion from the camera.

I agree with one of the prior answers, that is likely a replacement bolt. If you're lucky you have a Bike Co-Op near you to hopefully find a replacement. Sometimes a seatbolt can work. Grind a little flat and you'll be set.

Or simply shove a thick screwdriver in there while you turn the nut.

Good Luck !