IIR, The western parts of old Russia were considered part of Europe. When the USSR came on the scene, the demarcation began to move west. Then with almost fifty years of the iron curtain, it became the norm to partition the area based on form of government.
PS: Yep I just looked up Europe in a 1929 edition of the Encyclopedia Britannica. And I quote:
“Europe within the narrowest of physical limits (to the crest of the Urals and the Manych depression and the Caspian Sea,----)”.
About a 1000 words on the boundaries, but I’m not going to type them all.
