M3 DOF in viewfinder?

[Dave Munroe <dmunroe@hpvclmun.vcd.hp.com>]

I've seen several references to a quick way of assessing depth of field
in the viewfinder of the M2 and the literature implies this is true of
the M3 as well.

The diagram shows what appears to be the central rangefinder patch, with
different size "notches" at the top and bottom of the patch.  From what
I remember, the near and far limits of depth of field can be determined
when the double image of a vertical object falls within these notches.

The problem is, my M3's rangefinder patch is a quite ordinary rectangle
with no such notches.  They aren't referring to the gaps in the top and
bottom of the frame lines, are they?  If so, these gaps seem to be the
same width and, being outside the rangefinder patch, no double image
will appear within them, of course.

This has been puzzling me for quite some time; my grateful thanks for
any answers.

- -Dave