There's a quick view of the Cyclo one at 12:00 onwards in this Jack Taylor film:
It had two little solid convex side wheels to keep the lines good and straight along the tube.
They turn up very rarely at cyclejumbles and on eBay etc:
I've not seen this facility on any other design, just a single arm for following an edge, although it should be easy enough to fabricate something to do the job on a Beugler unit or such.