How did the grandfather treat the cop who found the zither?

Grandfather, believing the cop was a deserter, shot him.

"Back t' the lines, ye goddam lily-livered cattle!" With that, he fetched the officer who found the zither a flat-handed smack alongside his head that sent him sprawling. The others beat a retreat, but not fast enough; grandfather grabbed Zither's gun from its holster and let fly. The report seemed to crack the rafters; smoke filled the attic. A cop cursed and shot his hand to his shoulder. Somehow, we all finally got downstairs again and locked the door against the old gentleman. He fired once or twice more in the darkness and then went back to bed. "That was grandfather," I explained to Joe, out of breath. "He thinks you're deserters." "I'll say he does," said Joe.