Incorrect..... the US6 wasnt introduced into service until the end of 1943, by then the Russians already has over 57 regiments of BM13 based katyushas, I think you are confusing the figures because the US6 remained in production till the late 1950s in a variety of roles where as the BM13 was withdrawn from service in 1946 and handed off to the Chinese and koreans.
DMz2010
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