2001 and 2003 are largely the same with one big difference...
The 2003 has a 3-micron 797 audio single-diaphragm capsule, the 2001 has a 6-micron Shanghai capsule.
The difference is very much in the 2003's favour: significantly reduced HF lift around 10kHz, smoother extension at both ends of the spectrum. -If you were going to mod either mic, make it the 2003. -the 2001 can be greatly improved, making it a very usable microphone, the 2003 actually becomes a very GOOD microphone with the Royer mod.
Keith