Argus As III (O) aircraft engine, produced under license by <a href="https://en.wikipedia.org/wiki/Opel" class="extiw" title="en:Opel">Opel</a> during World War 1, induction side.
Air Ministry, United Kingdom
<a href="https://en.wikipedia.org/wiki/Robert_Brooke-Popham" class="extiw" title="en:Robert Brooke-Popham">Robert Brooke-Popham</a>: <i>Report on the 180 H.P. Opel Argus Aero Engine.</i> H. 756. Enemy Engines Report No. 19. Air Ministry, Directorate of Research, 1919.