After reading the link Shaun put up i now understand what that 'thing' is for
The Portacatena was introduced at the '77 Milan show, and made it's first catalog appearance in 1979.
Used with a version of Campagnolo's 1010B dropout taped with two screw holes, the portacatena consisted
of an aluminum rail that attaches to the inside of the dropout, and a special rear shift lever. The idea is for you
to replace your 6 speed cassette with a 5, the aluminum rail attached to the inside of the dropout takes the place of
the 6th cog. In the advent of a puncture you shift over to thee rail which should hold the chain out of the way and
make for speedy wheel removal. When I install my portacatena, I'll add more detailed photos of it on the bike and in
action as well as a better description of it's function.
I hate RED DWARF