Gain of each stage? It's not quite that simple. The current flowing affects the gain as well .
This EF91 is running well outside the datasheet range - at a very low anode current. It won't be linear ( but who cares )
I'd guess that the gm is probably in the region of 2mA/v, which gives a gain of 160ish .The output of 2 stages will be pretty much rail - to - rail with 15mV p-p input.
I suspect ERNIE used that as a logic input to a timer/counter stage - the counter would overflow many times to give you a modulus at the end of the timer period. Perhaps they used the mains input frequency as a timer reference?
If my guess is right, my idea of using a schmidt and timer is closer to the ERNIE concept than using a/ds...............
Build one and see!
regards
Allan.
ps if you want a more linear design using EF91's with an output in the 5v peak-peak range , I'll do some sums. When I was young, valves were all we had!