The mid-30's as threej's states was the cutoff point. You can find many Finn Mosins dated into the 40's and even the 1960's and early 70's with hex receivers, but of course all Finn receivers were reworked old ones.
I know of no evidence that the round receivers were actually weaker than the hex. The high wall wartime soviet production were certainly *cruder*, but the legends of mass rifle shortages popularized by "Enemy at the Gates" were for the most part complete hogwash. The Soviets had plenty of rifles by 1941, and did not run out of them. Nor is there any evidence I know of that they had to resort to inferior qualilty steel or make dangerous cast iron weapons such as the last ditch Japanese or volk German weapons.
What they lacked in the early years were sufficient numbers of semiauto and full auto weapons from subguns to machine guns, along with anti-tank weapons, aircraft etc. These shortages were remedied by 1944 and '45, and by that time entire units were equipped with excellent subguns and semiauto rifles. By '45 they had a military force that was rivaling ours in mechanization and use of modern small arms. It's a good thing we decided not to fight each other then. Patton's brass ones aside, it would have been a really nasty fight.
