The 2 different length set of screws are to accommodate different door thicknesses. The inside and outside letter plates are sleeved and telescopic allowing for a door thickness range of 37mm min. up to 78mm max. (you could go wider but then there would be a gap between the 2 halves and your mail could snag or even worse disappear inside the door if hollow PVCu door). Select the screws which best match your door. If needs be the longer ones have 2 'waisted' sections allowing you to cut these shorter with a hacksaw or bolt cutters (make sure the screws threads are not damaged before screwing in to the bush)
The half with the double brush strips goes on the inside of the door.
A simple guide to installing would be as follows:
a. Jigsaw or hand/padsaw
b. Ruler/tape measure
e. 8mm dia drill bit
f. optional - hacksaw or bolt cutters if you need to shorten the longer waisted screws.
1. Mark the centre line of the door.
2. Mark out for the aperture required 262mm x 42mm check this against the larger of the 2 sleeved letter plates before cutting (remember the old adage - 'measure twice, cut once', to avoid irreversible mistakes!)
3. Drill starter holes in the four corners.
4. Cut out the aperture using a jigsaw or padsaw and clean up the hole to ensure the sleeves pass through. Protect the surface of the door during cutting if needs be e.g. use masking tape.
5. Mark out for the 2 No. holes for the fixing bushes at approx. 280mm centres, hold the outer plate in place and mark the position of the bushes on the door. (note: if preferred you could cut a slotted opening from the main aperture to go round the bushes, but I prefer the drilled holes as easier to do and holds the letter plate in position better).
6. Drill a min. 8mm diameter hole right through the door to suit the 2 No. bush positions
7. Fit the outer plate into position.
8. Fit the inner plate (with the brushes) into position, ensuring the gasket is in place and flat.
9. Open the inner flap, insert the suitable length screw through the centre hole and screw this into the bush on the outer half, repeat for the other side and tighten both screws.
10. Job done - easy and with minimal DIY skills and tools!