The 400D (left), 400H (center) and 400L (right) were an immense success for HP and were in production for about twenty years.  The circuitry for the three is very similar.  They are RMS AC VTVMs with frequency response from 10 Hz to 4 MHz.  Full scale ranges are from 1 mV to 300 V.  Input impedance is 10 MΩ, and the amplifier output terminals can be used to feed an oscilloscope or other instrument.  The 400D has a linear voltage and logarithmic decibels meter and a basic accuracy of 2%.  The 400H is essentially the same instrument with a larger mirrored meter providing 1% basic accuracy.  The 400L has a linear decibels and logarithmic voltage mirrored meter and 1% basic accuracy.  The 400H and 400L meter faces were custom-calibrated for each instrument by HP.

The 410B is an all-purpose instrument.  DC voltage ranges are from 1 V to 1000 V full scale.  The built-in tube diode probe provides AC measurements (peak reading but calibrated in RMS) from 20 Hz to 700 MHz in ranges from 1 V to 300 V full scale.  Indications are obtainable to 3 GHz.

Here's another useful instrument.  It measues positive and negative DC voltages from 1 mV to 1000 V full scale at 1% accuracy.  It is also an ohmmeter covering 0.2 Ω to 500 mΩ in nine ranges.  The ammeter function measures positive or negative currents from 1 μA to 1 A full scale.