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According to Wikipedia:
Compared to the standard Z3, M models featured a limited slip differential, a wider rear track,[14] and larger brakes (that are shared with the E36 M3). Z3M models were available in M-specific colors, they feature more aerodynamic wing-mirrors as well as redesigned front and rear bumpers and bespoke "Roadstar" Style 40 wheels, revised side gill and quad exhausts. The interior can also be differentiated by the voltmeter, clock and oil temperature gauges in the center console, leather-wrapped center console and door pulls, as well as unique M-styled seats and interior color options.
Do they all have the same bodywork? For e.g. would an LHD 2.8 have the same shell/wings as the M version?