This is a #10 x 1/2" length philips panhead screw. 

I sell these individually and in packs of 8. This being a #10 is a little thicker threads than the other screws. These work great for a variety of installations from mounting the speakers in your door or even mounting backstrap to the crossovers. These will usually bolt in to the back of most crossovers as you can see in the picture.