In this period there were several proposals to mount a heavy howitzer on a tank chassis to deal with heavy concrete defensive fortifications.
One of the proposals was to mount a 203mm B4 howitzer on the chassis of the KV-1 tank. The project didn't make it past drawing phase and was rejected for obvious reasons. - (The CAY-19/SAU or KV U-19 was a project of 1943 from "Lev Gorlitzky" who drew up this project. He received among others twice the “Order of Stalin”)
Model built by Axel Wech
Russian 1941 project with 107mm Zis 3 gun on the chassis of a KV-220
KV-4 project 1941 is the Duhov proposal - (several designers were asked to submit proposals regarding the KV 4)
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