I looked under "Adjustment of Height of Cut". I went to turn the black knob on either side, the ones you've put the red arrow towards. The left one, the knob just unscrewed right off the screw. The right one didn't budge. Are they what I'm supposed to be turning? If so, it seems like they've totally jammed up.
These knobs are just the locking screws for the adjustment. It should be possible to loosen them; if not, it's time for some penetrating lubricant [CRC 5-56, Inox/Lanox, Penetrene -
not WD-40].
The quadrant shaped 'roller brackets' should be free to move, but don't look like they are. These provide the actual cutting height adjustment. Some more penetrating lube, and probably removal to clean up rust between these brackets and the chassis, is indicated.
