What is the difference between dry operating weight and dry operating mass?
Weight is a force.

Mass is a property of matter. Acceleration makes mass produce a force. Specifically, a mass in the gravitational field of Earth effects a weight force on its support. The relation between both is the gravitational acceleration g, which is standardized to 9.80665 m/s² (approximately 32.174 ft/s²), as in Weight = Mass · g.