I see JR beat me..... see if we agree on anything.
First: the circuit shown uses Germanium diodes. It will work with Silicon, perhaps more predictably, but at higher voltages.
I started with the ass-umption they were Silicon and I'll plow onward.
D1 C1 D2 C2 form a peak-to-peak rectifier. The P-P voltage is held across C1+C2, bled by P1+R4.
Signal won't pass through the "ring" because C1 C2 shunt it all to ground.
R3 passes signal to the output.
R5 D3 D4 R6, with C1 C2, form a variable impedance shunt to the signal coming through R3. When the C1+C2 voltage is zero, the shunt is near-infinite, no attenuation. When diode current is 0.1 microAmp, each diode acts like 300K. So the shunting is like (300K+22K)/2= 161K. This will give significant attenuation against R3 470K (about 12dB).
The input level for this point is around 2V-2.4V p-p for Silicon, 0.5V-1.0V for Germanium.
When diode current is 1 microAmp, each diode acts like 30K. Ah, but there's 22K in series with each. So the shunting is like (30K+22K)/2= 26K. And when diode current is 10 microAmp, each diode acts like 3K, the shunting is like (3K+22K)/2= 12K.
The plan shows 10V input, 28V p-p. This would cause almost 14V/22K or 500 microAmps in each diode, which should drop to 60 ohms, but the 22K series resistor has been limiting the limiting (hah) since about 1uA diode current. So it would seem to max-out at 32dB limiting (which is pretty good for the simplicity).
Yeah, it will work, and is probably pretty optimum for a thing that does so much without amplifiers. The input charging current is brutal, and the output impedance is awful high and variable; that's what comes of avoiding amplifiers.
I *suspect* this goes between a TV set speaker (good low impedance) and the hi-Z mike input of a PA amp, to knock-down loud commercials so listening is tolerable on a better speaker. So while "passive" it requires a loudspeaker-like source and a 4-tube amplifier behind it; but you may have such gear available and the limiter itself is simple and un-powered.
These days you can put a resistor and LED on the gitar amp output, shine on a LDR and resistor at the guitar input or some low level stage.