Locking the steering box (not necessarily the wheel) in the absolute center point is one of the places where a special tool, i.e. a pointed bolt, is used. There is a bolt on the steering box that is removed and the pointed bolt is screwed in a detent in the steering shaft. This is what locks the box in the exact center position and everything is adjusted off of that. Hopefully when this is done, the steering wheel is also centered, but it is possible that a shop didn't do the alignment right the last time and even though the steering box is centered, the steering wheel may not be.

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Len
'59 220S Cabriolet-SOLD and living happily in Malta
'83 240D 351,500 miles original owner-SOLD
'88 560SL 41,000 miles - totaled and parted out
https://sites.google.com/site/mercedesstuff/home
'99 E300 turbo 227,500 miles
'03 SLK320 40,000 miles - gave to my daughter
'14 Smart electric coupe 28,500 miles
'14 Smart electric cabriolet 28,500 miles
'15 Smart electric coupe 28,000 miles