To disassemble the Geiger counter, you first have to remove the knob and axle of the rotating drum, then the drum itself, and then tilt the double PCB up from the rotary contacts side. (I already desoldered lots of thin wires from the analog meter movement)
The aperture is supposed to open all the way but the clutch that engages the knob (yes the knob, not the axle) got bent because it's not visible from the outside that it's even there.