The 85 mm anti-aircraft gun introduced into service by the Russians in 1939 was an acknowledgment that their existing guns were not capable of meeting the demands that would be made on them in the coming war.
The Model 1939 was a scaled-up variant of the Model 1938, and was often referred to as the KS-12.
By 1939 all guns produced in Russia were intended for use as anti-tank weapons as well and the Model 1939 was later revised to become the main armament of the famous T-34/85 tank.
The Model 1939 can be regarded as one of the most successful Russian guns for it is still in world-wide use today.
In 1941 the Germans were glad to use many captured examples as the 8.5 cm Flak M.39(r) and like other captured Russian guns, they were rebored to become the 8.5/8.8 cm Flak M.39(r).

Special thanks to Mr. Yuri Pasholok, Russia for providing the photos.