|Three Swedish stamps|
These Swedish stamps were issued in 1987. The picture on the centre stamp depicts a Swedish performer by the name of Reino. It was mentioned in September 2001 in the newsgroup rec.sport.unicycling that this Reino would have accomplished in the early 1960's to ride an Ultimate Wheel with his hands on a slackrope. Nothing short of amazing! On the stamp the wheel seems to have a tyre (even more apparent from larger picture) but in reality he performed this trick with a hollow rim.
The Swedish unicyclist Klas Axelsson met the then 82 y.o. celebrity in 2003. Reino told him that he practiced nothing but that single trick for six years until he could do it, and that as far as he knows, he's the only one who's ever done it.
There is some more information about this trick (including photos) on a website dedicated to 'old' Swedish performers.