If you have a 420 then it's highly unlikely to have the hydraulic suspension unless it was a special order. Do you have a switch on your dash that looks like a little shock absorber? If not, then you have the standard suspension.
The rear most likely has self leveling suspension, but the front does not. H&R sells the correct springs. Be prepared that you may have to “play” with different thickness spring pads in the front to dial in the spot on ride height.
Are you doing the work yourself?
Open the hood & see underneath the cover you see in pic attached . If the Reservoir for Hydraulic oil is there ,you got SLS Suspension if not ,normal Struts . Pic provided by ISS.
2nd , On a car with SLS suspension , you only need to Lower the front since the back has an adjustable Lever for Higher & Lower .
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