Install your chosen lock or latch. Dismantle knobs, offer the sprung back plate up to the door and align central to the spindle hole in the door, mark position of pins. Using an awl or small drill make two holes in door just smaller than back plate pins, tap back plate into position, it should hold itself to the door. Slide the spring over the spindle connected to the fixed knob. Insert the spindle through the back plate and door and out the other side. Select the second back plate and install as before. Slide the collar over the spindle. Screw the final knob onto the spindle applying pressure to the sprung end of the knobs. Engage the pins in the collar with suitable holes in the threaded knob. Release the spring tension (let go of everything). Hey presto! Super high quality door knobs installed with nothing (apparently) holding the back plates in position and no fiddly grub screws to lose. Long threaded spindle suitable for doors up to 7 cm thick. Some very minor surface character. Search espacenet for GB354190 1. Improved knob and spindle fittings for doors and the like Patent date 1930.