Established 1916. Reorganized as Northrop Corp., a subsdiary of Douglas Aircraft in 1932. Spun off as Northrop Aircraft Inc., 1939. Renamed Northrop Corp. in 1959. Merged with Grumman in 1994.

Jack Northrop bio from Aerofiles.com.

One year pin.
Another one year pin.
Three year pin.
Five year pin.
Another five year pin.
Ten year pin.
Note that this pin (and the following fifteen year pin) start to show a transition to the new style pin.
Fifteen year pin.

Why the change from the beautiful old-style pins with the graceful, almost deco-looking bird to the hideous new-style? Ug.

Unknown year pin. A one or a three year pin?
Five year pin.
Ten year pin.
Fifteen year pin.
Twenty year pin. The (two) diamonds have been removed from this pin.
Twenty five year pin. The (three) diamonds have been removed from this pin.

Unknown year pin with 1 white and one blue stone. If I had to guess, I'd say this is a 15 year pin.