I have a new to me but not new sidelock which is in excellent condition. I have shot probably 5 boxes of shells (1 ounce 1150- 1170 fps) thru this gun. I notice the screw on the left sideplate has moved, loosening. Moved from the 9 o'clock position to about a 7 o'clock position. The screw seems tight, hard to turn. Does this screw hold the sideplate in position or is there other functions? What is the significance of this change?