I think this includes the states of Warsaw PACT. Czechoslovakia never border Russia, or the former borders of the Soviet Union. In fact, It was forced to be communist, and is considered today to be a Central European country, and some people can't decide if it's eastern or central.
I think it'd be helpful if you gave a specific time period this is coming from. The USSR existed for ~80 years and borders changed during that time. Estonia, Latvia, and Lithuania were unoccupied before WWII along with Tuva, Ukraine was briefly independent when the civil war was happening, and around WWII Japan held land area in Manchuria that would border the Soviet Union.
Japan and USSR also at various times in the early 20th century had a border along the 50th parallel on Sakhalin Island. The USSR invaded south Sakhalin in the final months of WW2 and has occupied it ever since though (now as Russia).