The part you refer to is the release lever that the Campy chain hanger RH shifter incorporates. Actuating the sprung domed lever allows the shift lever to move past its normal stop, thus allowing the chain to derail onto the alloy chain hanger that is bolted to the rear right dropout.
Marc Boral Long Beach, CA
From: Craig Sandvik Date: Monday, May 20, 2002 4:02:27 PM To: email@example.com Subject: [CR]Mystery shifter part
What is the domed hinged part attached to the right downtube shifter on this Della Santa?
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.