US WW2 1st Lt. pin back rank insignia

American Officers full size First Lieutenant (1st Lt.)  rank insignia.

This is the early war style or sometimes called the British made style as it has a pin back fastening.

The pin back fastening is ideal to wear on the shoulders of the A class dress tunic as the end of the epaulettes are usually sewn down making the fabric 2 or 3 layers so the standard clutch back fastening rank badges will not go through that many layers of fabric. The pin back fastening simply pins through like a safety pin. 

The rank can also be worn on the Officers shirts and Garrison