Popper 15 Easy Studies (G. Woerl)
IMC# 2551 15 Easy Studies edited by G. Woerl has a Cello II ad lib part for Studies 1-10. This book is the precursor to Op. 76 and Op. 73.
*There is another edition based on this one, edited by G. Woerl & Carter Enyeart which adds the Cello II part for the rest of the studies. IMC# 3705