I don't believe that the Josef Slonski that you found in the casualty index at the Kramerius site is your great uncle. The Josef Slonski that is listed was born in Zurowa, Jaslo, Galicia in 1874. He was in k.k. LstIR Nr.6, in the 11th Kompany. He was captured in Sulmona, Italy in 1917 and made a prisoner of war.