In the 1440/1640 vs the 1460/1660 the differences were drivetrain/front axle, engine, grain tank extension.

The second year that the 1644 was out they started using the same transmission, axle and final drives that the 1666 was using.

I'm guessing it was cheaper to use the 66 components rather than keep manufacturing specific parts for the very limited number of 44's being built.