Had to change the rear left brake light today, and yes, the top screw dropped into the abyss. I did not have a spare screw. Determined to get to the bottom of this issue, no pun intended, I tied a length of string to a small socket and dropped it from where the screw fell.
The socket hit a ledge of metal above the bottom of the bumper on one occasion and on another it dropped down to the ground. I then tied a magnetic socket holder to the string and repeated the experiment. After the third attempt the missing screw was attached to the magnetic holder.
Success and the light cluster is now properly secured. In hindsight I think that it is best practice to partially unscrew the top screw and withdraw the light cluster slightly to hold the screw in place. Then the screw can be fully unscrewed without the possibility of it falling into the abyss.
Talking about these rear tourer lights i have only today been expreimenting in the shed on how to reinforce the brackets because as most tourer owners know these can break, also the ball socket fitting are a nightmare if a bulb happens to go just stating a thread up on how i went about the reinforcement Arctic