||Your cover was examined under the German Exchange Control rules which became law on 15 November 1918 when wartime censorship was stopped. Some 25-30 wartime censorship offices converted to exchange control offices. Office 4 (the number on the label) was assigned to Dresden Postueberwachungsstelle (Postal supervision Office) which had responsibility for mail between Germany (excluding Berlin, East Prussia, Baden, Wurttemberg, Bavaria including the Rhineland Palatinate and Darmstadt)and Austria from about Jan 1920 to November 1923. The number 37 may be an express delivery cachet of Leipzig.
I have written a number of major articles on the subject which you will find in Germania but the Dresden one needs updating.
The Exchange Control Inspection Office at Dresden 1918-1923, vol 19, no 3, June 1983.
Little Known Inspection Offices 1918-1923, vol 21, no 2, April 1985.
The Difference between Exchange Control Supervision and Examination Offices, vol 22, no 2, April 1986.
German Exchange Control - Standard Sealing Labels, vol 23, no 1, February 1987.
Munich Exchange Control Office 1918-1923, vol 23, nos 4,5, June, September 1987.
Hamburg Exchange Control Office 1918-1923, vol 26, nos 2,3, April, June 1990.
Berlin Exchange Control Offices for the Letter Post 1918-1923, vol 28, nos 3,4, August, November 1992.
The Emmerich Postal Supervision Office 1918-1924, vol 32, no 4, November 1996.
German Exchange Control at Karlsruhe 1918-1924 and the French Censorship of Mail in Alsace Lorraine 1918-1919, vol 37, nos 2,3, May, August 2001.
Konstanz Exchange Control Office 1918-1924 vol 43, no.1, Feb 2007
Königsberg Exchange Control Office 1918-1925 vol 45, nos 3,4, August, October 2009
Most of these have been translated into German by W Vogt and published by the ArGe Zensurpost