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.
The short answer is there is no official symbol. During the imperial period until about the early reign of Nicholas I around the 1820s, a symbol with the Russian letters "er" (Р) and "u" ( У) was used, pictured at right. It has been replaced with a variety of symbols including 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.