I suspect you're thinking of the planned partition of Prussia if it was decisively defeated in the 7 years war, which nearly happened. As I understand it Russia would have taken E Prussia and traded it to Poland for the Duchy of Courtland.
Prussia was nearly defeated by 1762-63 so it could have happened. Especially since with the fall of Pitt Britain was being less supportive. If so I think you would have gotten a Russian annexation. Most likely to trade it off as Courtland would have been more convenient for them.
The main key to having this happen is to prevent Peter III of Russia from coming to power. His stint was the reason why Russia changed sides. Heck, the Russian Army was inside Berlin, for crying out loud.
