[Leica] depth-of-field vf indicators?

[emu@aloha.com]

Years ago, I had a copy of a Leica book that explained how some M model(s)
had visible in the viewfinder something that functions as depth-of-field
(depth of focus?) indicators for two f stops, maybe f8 and f11. The idea
was that you compared the distance between the double images of some
out-of-focus part of your subject with the size of these two thingies to
learn whether that out-of-focus part will, in fact, be photographed
sharply.

The book is long gone, but I've now got an M2 body with peculiar little
extensions from the top and bottom of the bright rangefinder focus patch I
see in the middle of the finder. The little extensions are two different
sizes. I suspect these are what the book was talking about. Can anybody
explain?

Many thanks

Ernest Murphy