122 mm corps gun M1931/37 (A-19) or 152 mm howitzer-gun M1937 (ML-20) in towing position, with breach at full recoil.

Stalinetz tractor available seperately
 
The A19 was a soviet field gun produced from 1931-1946, combining the carriage of the ML20 with a slightly earlier 122mm field gun. The medium artillery piece was pressed into service in an anti-tank gun, though the carriage was not fully suited to this role. The gun was latter used to arm ISU-122 self-propelled gun, while a modified version was used on the IS-2 and IS-3 heavy tanks.

The ML-20 was a soviet heavy artillery piece produced from 1937-1946. Efficient in both a indirect howitzer role targeting infantry and fortifications, and as a direct fire gun against armoured targets. It was also used to arm the SU-152 and ISU 152 self-propelled guns.

Both were used from Khalkhin Gol in 1939 onwards, it continued in Soviet service long after WWII. German, Romanian and Finnish forces used captured examples.

Master by Richard Humble.