Back in the days when the League of Nations was deciding all this, there was a road sign in the German Empire consisting of a red circle around a white backgrou

Back in the days when the League of Nations was deciding all this, there was a road sign in the German Empire consisting of a red circle around a white background with a black arrow pointing straight ahead (and no slash). This did not mean it's forbidden to go straight ahead, it meant it's _mandatory_ to go straight ahead - the thing that later on was changed to the white-on-blue arrow. (examples: https://de.wikipedia.org/wiki/Bildtafel_der_Verkehrszeichen_im_Deutschen_Reich_von_1938_bis_1945 https://de.wikipedia.org/wiki/Bildtafel_der_Verkehrszeichen_... but the sign must be older than that). If you want to be picky, the "you must" signs generally had a narrower red ring than the "you must not" ones, but even that rule is not always adhered to on the linked page.