Hi,

Since cars having this 'variety' in the running boards
are rare perhaps one of the following speculations
work ...

a) This was some kind of "test car" that was used to
compare the longevity or the quality of the footing
of the metal version - against the wood part.

b) Some other road replaced only the center portion
and used wood instead of metal - perhaps they
didn't have the correct metal type or something
like that.

c) Ice reefers had a 'known expiration date' in the
50's. The mech reefers had started to show up.
So "don't spend more money on the new tech
boards than we have to" (because the metal
ones will outlive the expected life of the car).

- just speculations on my part ... Jim B.

P.S. I use a color pic of a rail yard that looks down on
the tops of the cars as the background of my
computer desktop. You can pick out a lot of
different running boards in it. None of them
has this particular variant. And I've never
seen another shot of a car like this one.
Good Catch!