These are museum pieces, so must be taken with a grain of salt. Sheet metal rust easily maybe the parts aren't original. Heck the emblem could have been switched. The changes could have something to do with production dates. Maybe the N+B team can shed some lights on this. I would prefer to have more pictures and more angles to say something definitive or be able to see them in person.
Do you have any period pictures?
The Borgwards grill seems like the top portion may have a almost trapezoidal portion tapering back to the filler cap from the middle 3rd of the radiator grill stamping (note the shade and light). I kind of feel that the taper extends to the bottom of the stamping by looking at the sides of the stamping making the middle 3rd of the stamping more prominate but if it does them I am confused by the straight bottom portion I see nothing to indicate any transition between the straight bottom and the slight taper. The BW grill seems the have a larger radius around the outer front face. The upper corner radii are larger on the BW which may even continue into the radii into the hood to create a differnt radius on the hood. There are some other minor differences from production that would be barely vissible in 1/35. Overall the KM grill is more squared than the BW grill. I could see these changes made to speed up production or as just a difference in styling as enterpeted by 2 differnt manufacturers.
Improving my modeling one mistake at a time.