As I was busy in this part today I just took a look at my car; it's Swiss delivered, and my 6 acorn nuts on the strut towers all are chromed brass. The chrome is still shiny, however the mirror like reflection like on a bumper doesn't occur as the surface is too rough.
As my car has been restored once before I can't vouch for their correctness.
As I was thinking they were made from stainless steel or chromed brass, I tested them with a magnet.
As expected, they were non-magnetic. So I sanded the chrome layer of the back of the nut.
Turns out it is brass.
Interestingly, if you look closely at them, you can see the brass material was turned on a lathe. 3 of my 6 acorn nuts were turned soo quickly/roughly that the lines are still visible.
Also the 3 pictures from
@HB Chris of the European cars, one can see that at least some of these acorn nuts clearly have the milling lines still on them, with a distinctive point in the center:
In my pictures on my wooden table are also 2 stainless domed nuts, these are clearly a bit more grey. Also 1 simple zinced nut and a zinced domed nut ( with it's cap smashed in) for color reference - these are clearly more dull. These other parts are distinctly different in color vs the 6 domed nuts on my car.