[CR]Mystery shifter part

What is the domed hinged part attached to the right downtube shifter on this Della Santa?

http://www.campyonly.com/retrobikes/della_santa.html

There's also a nice shot of the Porta Catena. I think the trick use of this piece would be on road fixies with flip-flop wheels.

Curious,
Craig Sandvik
Berkeley, CA