The adjuster is calibrated so that when a blade is fitted and the top plate tightened, the knob on each Digress always lands on the same place, assuming the triangle and notch are orientated correctly.
The different knobs (Brass, Aluminium or stainless) all result in the razor balance and weight differing from the original Progress.
Whether they are worth £40 extra is an individual call, that is for any buyer to decide....
The work that goes into them, materials (allied to the risk of damaging a Progress beyond repair) does require an additional charge....
An individual can buy a Progress and opt to have it "Digressed" later!
Glad you enjoyed the shaves ?