The Symmar is a (nearly) symmetrical design with both the individual cells
(in front of and behind the shutter) capable of forming an image, and the
two together cancelling out (well, nearly) many of the aberrations.  A 'side
effect' of the symmetrical design is that it is also a "convertible" lens.

This means that you can remove one of the cells and use just the other as a
lens of somewhat reduced quality but longer focal length.  In the case of
the Symmar design the results when 'converted' are better with just the rear
cell than with just the front one, and you will find that the front cell
unscrews from the shutter with a sturdier screw-thread than on most lenses,
precisely because it is designed to be removed and replaced in this way.
(You can remove the rear cell instead and get almost the same focal length
as with the front cell removed, but at slightly less bellows draw.)  Symmars
in their original shutters normally have a second set of aperture markings
on the shutter for use when converted, and these are normally marked in
green, matching the green designation on the front ring of the lens.

With the 210mm f5.6, the converted focal length is 370mm, with a maximum
aperture of f12 - hence the markings you see.  There should be a second
aperture scale that starts at f12 too, but if not you can ask here and
someone will tell you how to find where to mark the scale.

I have this lens too, and at 210mm it is very good.  Converted it is less
so, but still very useable, and the converted lens is not a bad thing for
portraits.

Peter

The lens is a convertable lens.  If you remove the front element it bcomes a
F12 370mm lens.
PCR

  I've just gone over this thread again. I missed the part
about the aperture scale on the shutter starting at f/6.8.
Its not the right scale for this lens so probably the lens
was remounted in this shutter. Its not difficult to
calibrate the f/stops of a lens, I've posted instructions
here before but will do so again. Symmar lenses were
supplied in Compur shutters and were mounted without
adaptors so probably the cell spacing is OK. You can check
this by focusing on a very distant object and seeing how
sharp the image stays at the corners. If the cell spacing is
way off the marginal image will be quite blurry.
  The f/stops of a lens are not in general the physical
size of the aperture but rather the size of what is called
the entrance pupil. The entrance pupil is the _image_ of the
aperture as seen from the front of the lens. It is either
magnified or reduced by the front cell, depending on where
the front cell is positive (as in the Symmar) or negative
(as in a Triplet or Tessar). Where there is only a single
lens located behind the aperture stop, as in many
convertible lenses when only one cell is used, the aperture
stop _is_ the entrance pupil and its physical size is the
quantity you want. Usually, the marked focal length is close
enough to the actual focal length to use it for calculating
the stop. Just devide the focal length by the diameter of
the stop.
 To measure the size of the entrance pupil you will need a
white card with a small hole in it and a small light source,
a pencil flashlight will do. You also need a flat mirror
that will fit over the front of the lens. While a
first-surface mirror is ideal an ordinary shaving or makeup
mirror will do providing its flat (not magnifying).
  The first step is to focus the lens exactly at infinity.
This is done by "autocollimating", the lens acts as its own
collimator. A collimator is a device that emmits light that
is in parallel lines, that is, appears to come from an
infinite distance.
  Place the flat mirror over the lens. Place the white card
behind the lens with the flashlight behind the hole. The
mirror will reflect the image of the light back to the card.
Adjust the reflection so that the image is near the hole.
Focus for as sharp an image as you can get. The lens is now
focused exactly at infinity. Now, leave the card and
flashlight in place and remove the mirror. Place a
translucent screen over the lens, ordinary thin writing
paper will do. There will be a round circle of light on the
paper. This is the entrance pupil. Measure its diameter and
devide the focal length of the lens by it. This is the
f/stop. Once you have set this up you can calibrate a new
stop scale for the lens.
  For stops using only a single element on the back of the
shutter you can still use this method or simply measure the
size of the iris since, again, the physical size of the stop
and the size of the entrance pupil are the same when there
is no lens in front of the iris.
  There are a number of ways to make aperture scales
ranging from simply marking them on tape or paint over the
old plate (turn it around and paint the back for instance),
to having a new one engraved.
  You can use your view camera as a sort of optical bench
for making these measurements.
  This is a pretty old lens but its worth e-mailing
Schneider to find out if they have an aperture plate stuck
away somewhere. They can only say no.
  Schneider has a fair amount of information on
discontinued lenses on their web site so you may be able to
find one which shows the proper cell spacing or, perhaps,
the same e-mail could ask about that. If the new shutter is
the same as the original the spacing will be OK.
 I have one of these lenses. Its a very good lens and
performance of the back element by itself is reasonably good
at small stops. BTW, the front element can also be used by
itself. The threads of front and back are different, at
least on my lens, so you can't move the front cell to the
back, which is the ideal position for it. However, the
difference in correction between the two positions is small.
There is an advantage to using the cell in front: the cells
of this type of lens has principle points (where the light
appears to come from) located such that the lens appears to
be slightly retrofocus when in the "normal" position on the
back of the shutter, and slightly telephoto when in front.
Thus, the front position reduces bellows draw by more than
just the physical thickness of the shutter. It makes it
possible to use some very long lenses on cameras which do
not have enough bellows draw when the lens is in the "ideal"
position.