Had the "wing" on my old truck and my current one doesn't have it. Looking for the part number or what it is technically called. Here is a picture of it on my dad truck.
I have the original box for that spoiler/cap I bought for my truck. Liked it much better than the cap that came on my truck. Should have the part number on it. I will look tomorrow when I go to my trailer and get the number for you. Done forget that you will need 4 nutcerts and 4 of the those screws that go with it.
They have it on lmc truck. Cheap too
Those are the one that came on the earlier trucks Tom. I think the one he posted the pic of is from a 2006 +