Which of the following is not a diagnostic feature of gallstone ileus on a plain abdominal radiograph?
Characteristic feature of gallstone ileus is Rigler's triad :

Incomplete or complete small bowel obstruction; 
Gas in biliary tree; 
Ectopic gallstone.
Ectopic calcified gallstone
Stone < 2.5 cm size in the intestine
Signs of small intestinal obstruction
Gas in the biliary tree