The difference is as simple as the facelift or the M5. The e39 doesn't really have physically wider vs narrower grills like the e34 did. The only difference is the chrome surrounding the slats. The 96-98 kidney grills do not have an overlapping chrome surround that spills out onto the surface of the hood.
The e39 M5 was the first to get the "Angeline Jolie Pitt" kidney grill treatment...then when the facelift came along in 2001...all the grills had a surround that spilt out onto the surface of the hood.
The "Angeline Jolie Pitt" kidney grills (broad lipped chrome surround):
In the images of wrecked ee39s below...you can see how the kidney grills don't have the larger chrome surround that pours out onto the surface of the hood:
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