source: Russian battlefield -site again.
http://www.battlefield.ru/index.php?...d=78&Itemid=50
There is a line:
"In May 1943, the "T-34 tank-hunter" and "KV tank-hunter" were accepted for service again. They were armed with modernised ZIS-4M tank gun... "
So far that is the only info Ive found about it.
So it would look as the KV-57 was a similar thing to the T-34/57, the arnament was changed to a more effective (in AT role not HE) gun. These were limited edition vehicles. The vehicle ingame seems to reprecent this tank.