Actually,
The warpaint issue No. 32 on the Boston/Havoc shows a Russian A-20G with the standard .50 cal turret and 4 20mm cannons in the nose

Aircraft number 43-740 for your reference. On page 22 of the same book it states that, 'Russia received most of the early 250 early cannon-armed A-20G's.' Of course, it also talks about incorporation of the turret sometime after A-20G production would have begun. It does also say the cannon armed versions had just the twin .50s in the dorsal position, at least initially. So, is the painting on the back of the book incorrect, were some A-20G's actually deleivered to Russia like that? Or, did Russia mix and match a later A-20G variant with the cannon nose from an early model?
It also reports on the same page that the cannons were replaced because they tended to jam and they, the U.S., thought the rate of fire was too low.