A left lateral film (not shown) showed nearly same density of upper and lower thoracic spine conforming to some soft-tissue opacity in lower posterior zone of lung. Oblique film again showed triangular opacity with base over diaphragm and pointing towards hilum. Oblique fissure not identified separately.

Findings confirmed on CT (also not shown). 

Cause was left lower lobe bronchus stenosis with peribronchial calcification suggestive of chronic inflammatory disease (most likely tuberculosis in this subcontinent).