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