Jan and Waclav Zvolanek born in Jakubec? Takubec? Russia circa 1891-94

Grandfather Jan (John b.1891) and his brother Waclav (James b. 1894) were of Bohemian race, but born in Russian town of Jakubec or similar.

My grandfather’s last letter to family in 1917 was returned to sender. He lost track of his family after that.

I can not locate Jakubec Russia via Google. Perhaps it is no longer part of Russia, or no longer exists.

Can anyone help me determine where my grandfather was born? It would be an immense first step in figuring out the mystery.