Is the lower part of this sprung-loaded? If so, just wondering if it is some sort of twist-lock, off something. Must admit, it doesn't look OEM. Just a thought.
Looks more like the cruise control device which was an accessory in the early 1980's. A friction device fitted to the throttle grip to partially grip to the handle bars. similar to a sticky throttle. You had to manually twist the throttle back. a very useful accessory, i had them on both my old R65 and R1100GS bikes.
Thanks for the suggestions but mystery solved I believe, I managed to find the previous owners number and asked him. He then contacted his brother in the US who also has an R1150RT and he worked out that it is the part that holds the lid retaining strap to the OEM topbox. I don't use the OEM topbox anymore but I checked it anyway and the plastic lug that this part should attach to is snapped off which would explain why this part was not attached. I realise this is all a bit OCD but I didn't want to chuck it away and then find it was needed.
Funny thing is the US brother admitted it was him who snapped the lug when he borrowed the bike some time ago.
Any one with an OEM R1150RT topbox care to check theirs to provide final photographic evidence