Russian stamps from 1909-1917 were overprinted "Poczta Pol. Korp." (Poczta Polskiego Korpusu = Polish Corps Postage) for the First Polish Military Corps stated in Russia, named after general Dowbór-Muśnicki.
The First Polish Corps of general Józef Dowbór-Muśnicki was formed in Belarus in August 1917. The Polish Corps of Poles serving in the Russian Army was stated in Minsk. It was supposed to fight on the Russian side, but after the revolution it had to fight against the Bolsheviks.
Polish eagle overprints on perforated Russian stamps