The best source of information about the Nazi period Auswärtiges Amt insignia is:
UNIFORMS AND INSIGNIA OF THE GERMAN FOREIGN OFFICE & GOVERNMENT MINISTRIES, 1938-1945.by Jill. Halcomb Place Published: Columbia, SC Publisher: Agincourt Publishers Publish Date: 1984
Here comes some pictures illustrating the type of insignia worn
German Reichsaussenminister Ribbentrop
meets with the Russian Foreign Commissar Molotov,
in preparation for the Nazi-Communist
Pact of 1939.
Ribbentrop (with a later model of rank insignia) meets
with Italy's foreign minister count Ciano.
Werner Best, German plenipotentiary in Denmark during the German occupation.
The pictures are of senior officials in full dress uniforms, but the Auswärtiges Amt also wore grey everyday uniforms, and every level of officials from the lowest einfache Beamter
to the Reichsaussenminister
wore uniforms with rank insignia.