The display uses a simple RC oscillator to generate its internal clock. It will vary with voltage, temperature and from unit to unit.
Using an oscilloscope, I’m able to pick up radiated EMI pulses from the display that are emitted once per row refresh. From this, the screen refresh rate can be determined.
I have 3 different displays. Measuring their refresh rates under the same circumstances, I got 134.7Hz, 135.9Hz and 138.3Hz