One of the reasons my previous post may be significant is that it demonstrates that von Scherling kept issuing catalogues of medieval manuscripts during the hiatus between vol. III (1933) and vol. IV (1937) of his Rotulus series.
In 2015 I wrote a blogpost about the Dutch dealer Erik von Scherling, including an attempt to list all his known catalogues, especially the series with the title "Rotulus".
At my request, and through the kindness of Candice C. Brown at Duke University Libraries, two more of his catalogues have now been digitized, and are available online at Archive.org.