I’ve gone back and forth on this too. I initially thought that 1001XX and 1000XX were similar (if not the same) late 40s/early 50s cut with different denim weights, 1000XX being heavier. However, I now think that each cut may be slightly different and the double number suggests that it is, for example, a 1001XX cut made with heavier 1000XX denim, or vice versa.
But that’s just my take and I reserve the right to be completely wrong! 😆