The calendar sheet in April shows the billboard of the Klingenthal company Meinel & Herold, which was very well known at the time.
On the one hand, this documents the variety of products and, on the other hand, the export activities of the company. Meinel & Herold existed from 1893 to 1945 and was heavily specialized in the shipment of Klingenthaler harmonic products; it was a kind of "Neckermann of the accordion industry".
Interesting detail: The poster from the 1920s uses as a sign of trade between Germany and Brazil a flag in the colors black, white and red, the colors of the 1918 defunct German Empire. Black-red-gold were the new official colors of the Weimar Republic, but the merchant flag remained the old one.