It is the finder frame selector on a Leica M series camera body, this one specifically from an M9:
Much larger crop of the original image #17.
Two images from the M2 manual (gotta love the internet that you can find these things):
While it did change in shape over the course of the evolution of the M series (the tip is square on the M7 for example), the current lever is pretty close to the original.
As stated in the M2 manual, this lever lets you preview through the viewfinder the field of view of different focal length lenses when mounted on an M series body.
The function of this lever hasn't changed over time. From the M9 manual:
One of the fun things about an M series is that you get >100% finder coverage (unless you are shooting with a really wide-angle lens). This can aid in composition (though the finder guides aren't perfect--so it takes a bit of experience to know exactly what is going to end up in your image).