The NVM-1 is not available anymore.
The following publication offers detailed information about the NVM-1 nanovoltmeter (licensed by PTB):
Ultra-low Noise Chopper Amplifier with Low Input Charge Injection
Dietmar Drung and Jan-Hendrik Storm
Instrumentation and Measurement, IEEE Transactions on (Volume:60, Issue: 7, pp: 2347-2352), 2011
DOI: 10.1109/TIM.2011.2114030