Complex sentence: Through this period, further development of the T-34 was taking place and by 1943 engineers had succeeded in mounting the 85 mm gun to this chassis, making the IS-85 superfluous as the original T-34 had the KV-1.
Split into simpler sentences: Through this period, further development of the T-34 was taking place, and by 1943 engineers had succeeded in mounting the 85 mm gun to this chassis as well. This made the IS-85 superfluous in the same way the original T-34 had with the KV-1.