The Spyders use their rear shock as the 'limiting strap', so it's not one of those!
However, I reckon it looks a helluva lot like the 'earth strap' for the radio antenna! The RT's had their antenna high up there well clear of any real 'metal ground plane' on the back alongside the lid of rear trunk & behind the pillion 'arm-rest' - well, those pads where a pillion passenger might be tempted to put their elbows anyway.... IF they can get them on there!
Sounds (& from the pics, looks like it too! ) like you're having a lotta fun doing this Mega Good stuff, keep on at it! Watching your progress with much interest.