The statement in the Scott catalog is quite outdated. The printing technique for the ORN issues has been pretty well understood for some time. I suggest the following for information:
Lawrence, Ken. "U.S. Stamps That Went to War." American Philatelist, January, 1998. pp. 48-74.
Kloetzel, James. "From the Scott Editors." Scott Stamp Monthly. September & October, 2005; April, 2006; March, 2009.
The following is a really detailed, in-depth long-term study of the Luexmbourg-Netherlands issue (they were printed together), but also includes some information on other countries:
Crane, Clark S. The United States Specialist. Series of articles. June, 1974 through April, 1980.
The following catalog was published in 1945 (there were a couple of earlier editions) and has lots of info on varieties:
Engel, John. Engel's Flag Stamp Varieties Catalog, 1945. Liberty Circle, New York, 1945.
Also of interest are these:
Glass, Sol. "The Flag Stamps." The Stamp Specialist. H.L. Lindquist, New York, 1945 & 1948.
Hotchner, John. "Printing Varieties of the Overrun Countries Set of 1943-44 Stamps." Linn's Stamp News. July 19, 2010. p. 6.
Hotchner, John. "Overrun Countries Production Varieties; Unusual Packing Card." Linn's Stamp News. July 26, 2010. p. 6.
These are the key references I've used for my Gold medal exhibit on the Czechoslovakia issue. Some of the multitude of varieties are transient printing varieties; some are constant plate varieties. For the Czech stamp, there were at least three different printing plates used. The Czech stamp was printed with the Norway stamp (Czech in the top two panes, Norway in the bottom two panes). Czech varieties can be identified for each printing plate and/or for each pane position (left or right).
This series is a treasure trove for the dedicated "fly specker". I had a fabulous adventure working with the Czech stamp.