Ron Wisner recommends using a strong yellow filter (I imagine any
monochromatic filter would do as well) when using single cells of
his Convertible Plasmat set. This is to solve the problem of uncorrected
lateral color in a single cell. Now I do not use a #15 filter for that,
but I do use B&W filter 58ES MEDIUM YELLOW (022). Note that, when using
a single cell, the focal length tends to be quite long compared with
using a pair together.Of course, this would work only with black and
white film where blue light was not too important.

He presumably makes a corrector lens that has zero power but all the same
distortions so that it corrects a single cell. I ordered one when he first
announced it over 10 years ago, but he is yet to ship me one, so I cannot
say how well it works.

    The problem with all convertible lenses is that when a
single cell is used one loses the automatic corrections
introduced by symmetry or even semi symmetry. Symmetry
corrects the three lateral aberrations: lateral color, coma,
geometric distortion. Some convertible lenses, for instance
the Zeiss Convertible Protar, have some correction for coma
in the individual cells but most do not. For instance, the
Dagor, while patented and sold as a convertible really is
not because the individual cells have a lot of coma so work
only as very small stops (around f/45). The combined lens
has very little coma. While the cancellation is maximum
where the entire optical system is symmetrical (equal object
and image sizes) it is considerable even at infinity.
   The Plasmat is really a derivation of the Dagor where
the inside cemented element is broken free and air-spaced.
This gives the designer an additional surface and a spacing.
While the Dagor has considerable uncorrected spherical
aberration the Plasmat has very little. The design was known
for decades but not used widely because of excessive flare
where not coated. The availability of economical coating
methods following WW-2 changed the whole approach to lens
design making it possible to use air-spaced elements in
place of the previously used cemented elements. This
resulted in the practicallity of using some excellent
designs previously not used due to excessive flare and a
considerable improvement in lens performance.
    I don't know why the lens would need a color filter.
Perhaps its chromatic correction depends on its symmetry,
certainly its lateral color correction does. Filters have no
effect whatever on flare, other to increase it if the
filters are not coated.
    Process lenses when used for their original purpose in
making photo-mechanical half tone plates, do not really need
to be coated since the flare can be corrected by a small
change in exposure.