It's not AGC-related.
The BL-switch is the 'LOTS'-switch and it's shown in 'normal' position.
Push to activate 'LOTS' function.
They tap-off the picture carrier frequency and make a parallel resonant circuit with S63/C192A to peak the amplitude. X10 /C186 produces a DC voltage based on the amplitude of the picture carrier which of course varies with signal strength. A suitable level is fed to B19/Pin2 where the anode will vary accordingly with amplified levels. This amplified and signal-amplitude dependant DC-level is fed through R158 to the junction where it connects with the line oscillator pulse tapped off by C189/R152. The resulting pulse with DC imposed is fed to the other half of B19 into Pin 7 and amplified.
At the anode/pin 6 will now be a pulse whose width will depend on what DC-level was imposed at the grid-input. The valve may saturate earlier or later as the imposed DC is varied.
Superimpose that pulse over the regular signal by feeding it to the Picture-tube grid-one, the one that normally controls brightness. The pulse will turn-up brightness for it's entire width varying with signal-strength as shown.
Pretty ingenious
rgds,
/tri-comp
P.S.
Over the years I sold a LOT of DUX equipment where I worked in Denmark.
Outside different design from Philips; inside 99.9% the same as Philips models.