[William Sommerwerck]

The "rumble filter" circuit in this tuner is sort of interesting.
Rather than inserting a low pass filter in the audio section,
it changes the time constant of the AFC network. When the
time constant is lowered more of the "rumble" frequencies
are feedback cutting the low frequency audio response. This
means that the "rumble filter" won't work when the AFC is off.
Also the labeling for the "ON" and "OFF" positions of the
"rumble filter" switch appear to be reversed.

This falls into the "It's not a defect, it's a feature" category.

If the AFC time constant isn't long enough, the AFC will try to null out the
audio's bass.

I've never heard of a "rumble" filter for FM tuners, but it might have made
sense in the days when phonograph records were the principal program source,
and perhaps even during live concerts (when traffic and subway noise might
have intruded).