Frank,

It sounds to me like you are speaking of standard car siding. Most wood
caboose' with what looks like 2 1/2' "grooved" siding was actually 5" (plus
the tongue) tongue-in-groove planks with a V-groove in the center. The outer
edges are beveled so that when two planks are put together the edges form
another V-groove that matches the center groove.

http://i634.photobucket.com/albums/uu61/neoliticman/Carsiding.jpg

In 12" to the foot terms this stuff is available from lumber dealers today.
My last house had a small den with #2 (tight knots) car siding walls and
ceiling. Looked great but when I had to replace some due to a water leak
that led to window replacement, the new stuff was 1/2 " narrower than the
late 1940's planks used when it was built. So I had to make my own so that
the horizontal planks below the window line would be the same where they met
at the corner. The house was also built with 2X4's that measured 2X4. I
gotta tell you that room with a light (Pecan IIRC) stain then varnished car
siding looked just superb.

John Hagen