Later GM alloy wheels use “conical” lug nuts, all 3rd Gens appear to be M12-1.5. Some factory wheels came with black plastic covers on the lug nuts. If your wheels have those, the lug nuts require some course external threads to accept the covers.
Example (just to show parts - not an endorsement of vendor):
Premium quality 1982 - 2002 lug nut cover cap. Black in color. Replaces original GM part# 10028614 perfectly. Great part for your 82 - 92 Third or 93 - 02 4th Gen Firebird, also fits 76 - 82 Corvette. Sold each.
I was trying to understand what came on them to start with and did see somewhere about using plastic caps on them. If that's the case then it looks like I could also use the chrome 12 x1.5 conical based lug nuts, which Is what I think I will do to start with. I noticed that the holes on the wheels are flat with an indentation that looks like a flat washer might fit there. Thanks for the info.
The hole in the factory wheels should have a conical indented shape. Washers are used with ”shank” style lug nuts, not conical lug nuts. Shank style is often used on aftermarket wheels intended for drag racing. Are you sure you now have OEM 1987 wheels?
I looked at the third generation section of the showroom and they look like a few of the cars have the same wheels. There's a good picture on the 3rd page dated Nov 12 2007 under 1987 pontiac firebrd formula 350. Jesse.
Probably a good idea to check with those people to make sure you get the correct lug nuts. I jumped in to help because no one else responded. I have a 4rh Gen, and everything I know about the 3rd Gen wheels indicates they are the same as the 4th Gen. Maybe there ar some oddball wheels that I am not aware of.
The code in the console is N24 which are the 15x7 wheels which is what I believe these wheels are. I looked closer and there is a larger round recessed area around each hole but the holes are beveled for a conical nut. So the factory nuts would have had a plastic cap on them like shown in the pictures. Thanks for the help.