The Greenseeker Handheld sensor is manufactored by N-Tech Industries and sold by Trimble.
The sensor on the handheld unit and variable rate units are the same.
Fact Sheet PSS-2260: The History of the GreenSeeker Sensor
Development of the HandHeld "POCKET" SENSOR
The Pocket Sensor was developed to produce the same NDVI reading as a GreenSeeker just in a small more affordable package.