Why does my crystal resonate at 4 times the specified frequency?
This answer is actually @glen_geek's who commented on Jun 21 at 1:23:

That upper right knob &quot;VARIABLE&quot; looks rotated counter-clockwise. Should be turned clockwise to the &quot;CAL&quot; position.

and it is true. I have no idea why I didn't notice the discrepancy, I should have heard that 440 Hz is nothing like the tune fork and the 1 kHz of which I spoke was nothing at all like a high C. All my frequency measurements were 8-fold too high because of that variable knob.
And this also proves that this super-simple oscillator circuit with simply those 2 inverters is working well and reliable.
If Glen Geek @glen_geek wants the reputation I encourage him to put the answer up so I can accept it.