1st Triumph,
Hi !
First of all there shouldn't be a "Backing Washer" for the Battery cover !
There is a single 16mm long Button Headed bolt that takes a 5mm Allen Key :

The bolt screws down into a "Captive Boss" clipped into a slot in the Upper Fairing :

If the "Captive Boss" on your Trophy has been replaced with something else,
and you want to return it to original condition, this is the Part :

If it was me, I would want to find the part that has fallen down -
Who knows where it could end up being trapped - perhaps between a panel
and a wire or pipe which over time, with the engine vibraton, could wear it through !
Having dropped my fair share of screws etc down there, I find that they usually end up
either on top of the cover above the Radiator, or down inside the edge of the Fairing Panel.
This could be an ideal time to get to know your Trophy a little better,
and take the Fairing off - which you will probably need to do at some
point anyway, for any maintenance or adding accessories etc.
It's not a difficult task, just take your time, and note where all the screws come from.
Watch and listen for the missing part dropping down
onto the garage floor as you remove the panels !
This Topic will help, and also has a downloadable "Word" document listing where all the screws go ! :
https://www.triumph-trophy.com/index.php/topic,832.192.htmlCheers

