[Mondial]

Looks like someone was unhappy with the audio high frequency response of the radio and tried to modify the circuit. As you describe it, the mod would act to boost the highs a bit by placing a 50 pF in series with the volume control output. The parallel 400k resistor bypasses the 50 pF cap so the cap has only a limited effect. Actually, the circuit cuts all the lower frequencies so the net effect is an apparent increase in the highs.

As you noticed, the mod has very little effect as it only would relatively increase the highs by only 1 dB or so. Hardly worth the effort.