The only nerve agents I’m aware of are the G- and V- classes. The V- agents all seem to stem from work at ICI in the 1950s, and I can’t find any references to any one other than the Germans getting G- agents before the end of WW2.
Best guess is that you’re being sneaky and the “significant stocks” are the German Tabun stocks captured by the Soviets. As to what the agent they developed is, would a mycotoxin like “yellow rain” count as a nerve agent? That’s the best I can come up with anyway - this question certainly seems to have beaten Google
Edit: dammit, I can’t read - stocks of the agent they developed. Oh well, mycotoxins are still my best guess.