Even today I am not aware of the differences between FLUKE 8840A & 8840A/AF but I am aware of it similarities both has 0.005% accuracy on DCV when 8842A this has 0.003% .
Quick comparison of 8840A and 8842A this brings also the information that 8840A does not have 20mV/20Ω ranges.
Personally I have work with both models, 8842A this did not had ACV option card, this works nicely as soon it warmup as monitor of voltage reference at 10V DC, but due it age ADC noise level at last digit in comparison with Fluke 8846A this is night to day difference.
Fluke 8846A this is by far a better performer while is not considered as fresh technology within 2019.
About FLUKE 8840A I had the chance to repair two of them both with ACV option, while both get back at peak of performance, it did feel strange that auto-detect when input was AC Volts this was not working automatically.
You should select ACV function and then working voltage range and then to measure AC volts.
I got the impression that these meters operating software was designed (programed) by default priority that ACV 09 option card this will be missing.
When ACV option this is present, the meter does not include any Windows Plug and play detection software, and therefore ACV activation and range selection this is performed solely by user interaction.
You are reading these text lines because probably you are exploring market of used multimeter, either way now your are better prepared if any of those very old meters will serve your applications.