yo, that is the second generation of the first great Italian derailleur, the Vittoria Margherita. It is actually too “new” for that bike and I have since replaced it with the first generation VM, but more on that later.
The “lever” behind the crank is a chain tensioner. There is a pivot on the chain stay. The “arch” just above the crank connecting the seat tube to the down tube is a ratchet that holds the lever in place and keeps tension after you shift. The little triangle on top of lever turns and actuates the “flappers” on the chain stay between the crank and rear gears.
So to shift, you push forward on the lever, that makes the chain slack. While back pedaling, you twist the triangle which moves the flappers and then moves the chain to the next gear. You then pull the lever back to reapply tension to the chain. And Bob’s yer uncle.
The first generation VM does not have the flappers, you need to move the chain with your hand as you back pedal to change gears. It isn’t so bad when you get used to it.
For some history, the first year the Tour de France allowed derailleurs was 1937. All teams used the Osgear Super Champion, a French copy of the VM. In 1938, again all teams used the Osgear except the Italians (kind of a nationalist, fascist thing). They were allowed to use the VM. Gino Bartali won the tour that year using the VM.
Second gen
Frejus035 by iabisdb, on Flickr
Rachet
Frejus038 by iabisdb, on Flickr
Flappers
Frejus041 by iabisdb, on Flickr
A better bottle shot
_MG_9153 by iabisdb, on Flickr
My favorite detail, the chain oiler
