Steve/Rob you could be right as some sort of temporary knee protector? But it looks a very similar shape to the crown jewels protector at the crutch end of a Vokes filter.
I was going to post a picture of mine, but something disastrous has happened to my photobucket acc Ron

email (option): ronpier@talk21.com

There is one more thing with that photo, look at the location of the pillion footrest. That's not where they usually are fitted?

Henk

email (option): ahum@quicknet.nl