The first 10 Panzerkampfwagen III Ausf A were produced in May of 1937. Some sources state as many as 15. Only 8 of Ausf As were armed. They equipped units of 1st, 2nd and 3rd Panzer Division, and took part in Anschluss, take-over of Sudetenland and the Polish Campaign. Other unarmed III As were used for further testing.*
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Later in 1937, 15 Ausf B were produced. They saw service during the Polish Campaign. In October of 1940, five Ausf B tanks were modified and used as prototypes of StuG III series.*
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Also in 1937, 15 Ausf C were produced. They saw service during the Polish Campaign.*
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In 1938, 55 Ausf D were produced. Only 30 Ausf Ds, in 2 groups of 15, were armed. The other 25 unarmed Ausf Ds were used for testing. The armed ones saw service during the Polish Campaign and in Norway.*
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Their armor protection ranged from 5 to 15mm, and so offered limited protection against anti-tank rifle and machine gun fire, and virtually no protection against anti-tank guns. The reason behind this inadequate armor protection was a result of keeping the vehicle in designated weight range of 15 tons. Some of early models were up-armored and had their maximum armor protection increased to 30mm.*
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In order to keep the vehicle a secret, it went under several codenames: it was known as Zugfuhrerwagen (ZW) (platoon commander's vehicle), and it was designated Versuchkraftfahrzeug 619, Mittlerer Traktor (Medium Tractor) and 3.7cm Geschutz-Panzerwagen.