 92 Plant" bureau under V. G. Grabin, and AKB-43 (KB AU) under M. N. Kondakov.The "L-12" of Kirovskiy Plant was the first to reach ground tests (in April–May 1938), was returned for revision, tested again in August and given to the RKKA for further trials.Grabin's gun went through ground tests in March–April 1939 and was also given to the army.