Here is a copy of your specs to make them easier to discuss here:
None of those specs really tell you what the V/Hz of the generator inside that motor is. We could take a reasonable guess at if we knew that all the "Rated …" specs were at the same operating point, but that's not clear. What the spec calls "Rated" could be interpreted as "maximum", and the maximum conditions probably don't all apply at the same time. Note that (296 A)(176 V) = 52 kW, which is way more than the 20 kW power rating.
The easiest way to get some idea of the generator V/Hz is to run a motor unloaded at a known and fixed voltage and measure it's speed. For example, let's say we find the following:
1000 RPM = 16.7 Hz
2 Ω internal resistance
We know that the voltage drop across the internal resistance is (10 A)(2 Ω) = 20 V. The voltage across the pure motor/generator part is therefore (100 V) - (20 V) = 80 V. If there was no load at all, not even friction, on the motor, then the generator factor would be (80 V)/(16.7 Hz) = 4.8 V/Hz. In real life, the motor is still producing some torque to overcome friction and air resistance, so there has to be some small effective voltage left across the pure motor to allow producing the torque. Put another way, the generator can't be opposing all 80 V else there would be nothing left to turn the motor. Dropping the 4.8 V/Hz figure by 10% or so to 4.3 V/Hz is a reasonable guess.
A better way to get the V/Hz figure is to measure it outright by mechanically spinning the shaft and measuring the open-circuit resulting voltage. However, this doesn't work with all types of motors. In such cases, you can spin the shaft, then apply a varying voltage until the current is nulled out. That would be the open-circuit internal generator voltage for that shaft speed.
Is it usual to refer to "mechanical Hz" or "electrical Hz" when talking about V/Hz ratios?
If there is a V/Hz spec without any other qualification, I would take that as mechanical shaft Hz. For most purposes of using the V/Hz parameter, you care about the internal generator voltage as a function of how fast the shaft is turning. How that is implemented inside, including how many magnetic cycles the motor goes thru per shaft rotation, is usually irrelevant to the usage.