Reading the manual I have understood that after data loss in the NVRAM, you can do a simple re-calibration using the internal calibrator.
This can allow to make just qualitative measures. All linearization data must be re-entered with a complete calibration using very precise signal generator and power meter at different values of amplitude and frequency. This is necessary to get a good amplitude responce on the whole freq. range. The bad thing is that an elusive "271X Flatness Tests" original software is necessary to carry on this procedure.... and a working very old PC.
This is the reason why my hands are shaking to change a battery!