Just performed an oil change on my 996 (with the help of a very informative youtube video). Was noticing the car has very nice headers and what looks like stainless and titanium components stamped with Gillet part numbers. Was this stock, or is this an aftermarket system? Either way it looks very nice.

HJS makes the catalytic converters, not sure who makes the actual muffler, but if I had to guess what you saw was after market.

It is most likely your stock Gillet muffler. No reason to think it's after-market.

Porsche used both Eisenmann and Gillet as their OEM muffler suppliers. However both companies also make "aftermarket" versions as well.

I didn't really know anything about these cars when I bought this one, but I am having fun learning. I could see the pretty stainless mufflers, but once it was on the lift I was surprised at how nice the entire exhaust system is (I know, car geek ). I assumed it was aftermarket, but when I googled "Gillet" I mostly found take-off items on Ebay, which led me to suspect it was factory.