2001M also had B17 firmware version, which likely support 400Hz line frequency. Little birdie told me there is no RMS measurement different to usual 2001's.
Good point about 400 Hz line frequency as would be found on submarines, aircraft, etc.
2001M specifically supports RMS-calibrated average-responding AC measurements
in addition to the 2001's true-RMS and average modes. The difference is a correction factor applied to the average mode so that when used with sine waves, it gives the equivalent RMS reading. This is needed since so many military technical procedures are written for this type of meter response. The reading mode is selectable in the config menu for ACV and ACI.
Firmwares B16 and B17 (and maybe newer?) are for 2001M. Model 2001 then apparently goes up to B15. I am guessing that there is no hardware difference for 2001M and that the only actual difference is for the addition of the new AC mode, and maybe for adjusting integration time to 50 Hz mode in order to support a 400 Hz line.
It is worthy to note that the digital board schematic is for the version B firmware type (two DIP ROMs) not the older A type (one PLCC ROM).
I must be going blind. I looked at that auction several times but did not notice the schematics were included.
I have an eye for details like that. I was prepared to drop big coin on that meter just for the documentation, but then two things happened nearly simultaneously: TiN, armed with new keywords from those photos, found the docs with his google-fu, and the auction was pulled.