Undoing this screw allows the threaded wooden screw to be removed for adjustment and lubrication. For lubrication use soft wax or soap where it passes through the plane body.
More modifications: The clamp and plough screws are now retained by square hardwood (wenge) pins. These are removable for adjustment &c. as above. To remove, you will find a small hole drilled in the base of plough, in line with the wooden pin. Insert a suitable punch (with a squared-off end: a point-less nail works well); insert the punch in the hole and rotate the screw until the punch passes through the shaft. Gently tap the punch to push out the square hardwood pin.
The press cheeks have similar pins, which can be removed in the same way.