NEC NDV-21, NDV-22, NDV-23, NDV-24, NDV-31, NDV-100, NDT-41, RB-D41
This hack may also work on most other NEC DVD players
Make sure there is no Disc in the unit when you turn it on
Press Pause on the remote
Key in 3 1 4 1 5 and then 9 on the remote
The word Code should appear on the screen
Press the number for the required region (0 = multi-Region)
Press Pause
Turn the unit off then back on and you're good to go
Note:- Some new discs use enhanced regional coding which may not work with the "0 for all" setting. In this case use the specific code for the Region of the disc