It's definitely factory. It has the same style weld nut on the back as it does for the fender attachment. Also, I've seen others with the same identical hole with weld nut. This came off my early 67 car so it may be an aborted design feature but am trying to make sure. I believe mine had the same style phillips head screw as used for the fender attachment run down tight in the hole.
Camaro's didn't have this gusset as they had a filler panel in front of the hood. My car that these came off of is a February '67 car. My '67 parts car is a May car and it does not have this hole. The early car is a 400, but the X panels do not attach to this hole.
I have 3 67s,a sprint car coded feb,a 400 car date coded april and 326 ho car dated may.Just checked all 3,none have that hole.But that doesnt mean it wasnt factory.