i see. so i can apply TDK (1616N) firmware to the NEC 3500? is this advisable? will it remove the 'rip-lock'? also, if i apply an un-offical firmware can i rollback or flash with an offically firmware at a later date?
Yes, you can apply TDK 1616N firmware with no problem... but this will not remove riplock.
You can apply any unofficial firmware you want, and you can always rollback to any version (unofficial or official) you like. All you have to do is use the Dos Flasher...