i found this in a german forum and use babelfish for Translation:
Dear NEC team!
My brother has inadvertently on my NEC2510 in the DOS mode fir firmware of 2500 up play now it also as 2500 is recognized! I now the DL ability lost?
They must in the DOS mode the firmware of the NEC2510 .Da both models hardware Ident are, arise the DL ability only as a result of the firmware.