Also of interest: The double-headed eagle on the ruble was
adopted early on in the Rurik period from the Byzantine
double-headed eagle to symbolize the Rurik rulers' relationship
with the Byzantine royal family, the Paleologues, through
Ivan III's consort, Zoe (Sophie) Paleologue, a Greek princess.
R and руб. The Central Bank of Russia has been trying to decide since 2007 on a new design for the symbol and is considering both a P with two lines through it and the symbol below.
This post is adapted largely from Wikipedia. Thank you to Wikipedia, specifically the English-language article on the Russian Ruble for providing so much helpful information.