Chris is correct for the large Siler cock jaw screw. Although I just went and looked at some of my other locks and some other brands have different screw diameters and thread pitch such as --
1) Small Siler 12 X 24 --my jaw screw measures .210" in dia.
2) L & R double throat flint 1/4" X 28
3) Davis 12 X 24 my jaw screw measures .212