I've seen more fog lights than I could ever count,.... Never saw plastic brackets. I can confirm that there are in fact 2 different style factory metal brackets.
1 style is specific to the 85-87 Coupe nose
1 secific to all 1985 and up Z28 and similar noses
There are no Factory Fog Lights for the 82-84 nose. As a matter of fact - there probably aren't even any mounting holes pre-drilled in your front impact bar.
In order to mount factory Fog Lights on the front of my 84 I had to use the 85-87 Style Brackets and add spacers under tham to lift them to the correct hight. I also had to drill holes in the impact bar in the proper locations. A factory Fog Light harness from an 85-87 set-up ( independant harness ) was used and the system was tapped into the 84 headlight harness ( the same as the 85-87 set-up is. )
The result was that the system is - and works as - factory installed and the fog lamps are "hidden" behind the stock grills. Oh' Ya',..... I had to space the grills OUT a little as well !