Who among the men on horseback is my grandfather? I know he must be one of the four in the middle, because there is another photo of him in that kind of uniform (but no horse):
My grandfather to the right,
a friend of his to the left.
I'm hesitating whether my grandfather is the 2nd or the 4th man wearing that kind of hat in the "horseback" photo. Hard to tell as one face is rather blurry, and the other has his hat pulled down over his eyes. Both seem to have the kind of oblong face that my grandfather had.
Whether they had any choice as to what kind of military service to do, I don't know. But I know that my grandfather's grandfather (with whom my grandfather grew up) used to be an artillery soldier. Back in his day, military service was organized a bit differently, though.