88 Pontiac Sunbird catalytic converter

I bought the car a couple years back from my sisters boyfriend who owned it for 7 yrs and he had originally bought it off his mother who bought it brand new in 89, making me the third owner. And its never left the state of California. Unfortunately, while still in contact with the two previous owners, they have no recollection as to the nature of repairs on the vehicle in the past. Although, I was able to do a smog history search and found that its passed all but 3-4 smog checks out of about 16 since 1993. So that should answer your question about history of the car.
For the other question, the answer is Yes, modifications were made in the past. By the looks of the welds, I do believe this is NOT the original cat. What I do know is that when I bought the car, the muffler had been eliminated and it just had a straight pipe come off the cat and then was split to two tail pipes in the back, giving it a very loud racecar sound. I recently found the original muffler w/ pipe at a pick-n-pull and am in process of installing it. Which is why I was able to get a clear view inside my cat to find nothing. No honeycomb, packing, nothing. Clear all the way through. But I passed smog a couple years ago with the very same cat.