Leica M3 bright frame lines are much more sophisticated than just cut-out portions in a metal plate found in the Barnack Leicas.
Originally Posted by Jim Rice
In M3 the 50mm frame line (the one with the rounded corners) remains visible all the time. 90mm or 135mm frame lines are projected inside the 50mm depending on the lens You are using. The parallax error is automatically compensated over the whole focusing range.
The corners of the 90mm, 135mm frames (and RF patch) are not rounded, in fact they are razor sharp and that pisses some folks..
In M2 and recent M's, the 50mm frame line corners are not rounded, they are simply not present..