Aye Tommy,
If I remember correctly Smudge added a Squire chair to an XBR500 down in Wales, with the aid of Gareth et al over a weekend. The resulting creation was then ridden home to Dunfermline, where Smudge bided at the time. It being the first time that he had piloted an outfit!

Later it was moved on to I know not where.
I also know that somebody in Yorkshire, at one time made a trials/ enduro type chair that could be fitted to many bikes. When the XT500 first arrived in 1976ish I was working in the Isle of Man and one of the boys in the squad bought an XT500 and then had one of these chairs fitted. Great bit of kit! We went pass storming with it in the depths of winter over the Hardknott and Wrynose passes in the Lakes when nothing else was getting across in early 1978. Then went to Penrith and spent the rest of the evening after some Guinness and fish and chips, performing 3 wheel slides around the market square in Penrith in 4" of snow!

I suspect this current eBay Lot might be an inspiration!

Lot number: 202795640652 Link:
KTM-690-ENDURO-SIDECAR-OUTFITGood health, Bill