Well for sure it is some aftermarket with the duel bolt pattern.
It appears much like something we used to have back in the 90's that was a private brand wheel often made with a name and cap that is no longer around.
Often these wheels had caps with different emblems to change the ID of the brand. Generally they also did not have a MFG name on the back to make identifying it even tougher.
I think the best you can do is take the wheel to a shop and try to find one that will fit. Or you can take a Pontiac cap and see if you can modify it to fit your wheel.
I have some Prime wheels I modified a Pontiac Cap to give it a more OE look to the car. Many think they are factory wheels. I did have to trim them down and make some modifications. I also repainted them as they were the old red Pontiac hub cap centers. They came out nice.
The large number of wheels that came out in the 90's has left many people even with a name brand wheel scrambling for replacement caps as they often are discontinued and few are easily interchangeable.
I wish I could help more but I would recommend looking at other options as your best way to resolve this one.
Even if you find a original wheel or cap odds are it may not be in good condition either. Sorry.
[This message has been edited by hyperv6 (edited 11-07-2016).]