The genuine GM starter motor part number is 12567585 which were actually Delco Remy units.
(The original part number above has been superseded to 12575626 or 55556245)
Specifications:
Rated Power - 1.4kW
Number of teeth - 9
Length - 235mm
Bore Diameter - 11.5 mm
Flange Diameter - 75 mm
Position / Degree - rechts °
Connecting Angle - 50°
It looks like the Lucas replacement unit you purchased isn't the correct one, although finding a brand new genuine one is unlikely.
Bosch do a replacement unit, the part number is 0986019840 (about £185).
Hope that may help you. 🙂