Amateur drawing from the pre-war collection of the Viennese Institute for Sexual Research. The collection of this institute (and the older institute in Berlin founded by Dr. Magnus Hirschfeld) was destroyed during WWII

German text
Dienst auf dem WC:
Während die Herrin ihr Bedürfnis verrichtet, liegt der Sklave ausgestreckt auf dem Boden. Ist die Herrin fertig, so erhebt sie sich, macht einige Schritte vorwärts und beugt den Oberkörper über ein in den Raum zu diesem Zweck befindliches Möbel. Sodann richtet sich der Sklave auf und vollführt seine Dienste.
Wunschzeichnung eines Masochisten, die einem Brief an die Herrin beigefügt war
Nachlaß Dr. Alfred Kind — Archiv des Instituts für Sexualforschung in Wien

English Translation
Service on the Lavatory:
While the Mistress is attending to her needs, the slave lays stretched out on the floor. When the Mistress is ready she raises herself up, takes a few steps forwards and bends her upper body over a piece of furniture placed in the room for the purpose. Thereupon the slave gets up and performs his duties.
Wish drawing of a masochist, attached to a letter to the mistress
Estate of Dr. Alfred Kind — Archive of the Institute for Sexual Research in Vienna

I am very grateful for help I got by translating the written German text. And I very much like to share his view on the artwork:

As to what those duties are, we may speculate. The drawing suggests he might spank her, or even fist her, but surely this is unlikely? She’s holding a switch, after all. Much more likely, I suspect, his duty is to clean her in a manner acceptable to her.The grin on his face doesn’t help, as whatever is expected of him, he’ll enjoy it! Given what happened from 1933 onwards, we are lucky this quaint little piece has survived.