These don't look like Audi Cat converters?
 

I picked up a set of used pre-02 downpipes and cats for my k04 build. The guy I got them from gutted the primary cats but left the rears. Unlike real piggy pipes he didn't relocate the secondary o2 bungs downstream from the secondary cats. I crawled under the car to find the best location so I can relocate the bungs before the engine pull. The cats currently on the car don't look anything like the used ones I picked up. I am assuming the ones I picked up are actual audi oem and the ones currently installed on the car are not? can someone verify my assumption?

I was under the bottom of 3 C5 Audis (my 2001 A6 4.2, my current Allroad, and my brother's 2004 A6 4.2). They all had the same cats and they don't look like yours. Audi cats were flat.

Looks like someone replaced them. AFAIK. But they did a good job welding them in.

Other thing about them that crossed my mind is that they look much smaller than the audi cats. This would make me think they could be more restrictive as well...not too good for a K04 swap.

maybe the 2.8 is different but i dont have an"pre cats" just a downpipe from the back of the head that bolts up to a set of cats . the cats are flat on the bottom . The pic you posted are not OE cats .

Here's an interesting message that pops up on ETKA:

"on some catalysts the shape was changed from square to round. the round catalysts can be installed without any trouble."

But the message only pops up when you select the "exhaust pipe with catalyst" for the 2.4-2.8ltr


But I would agree that the original ones that came with your car were more than likely replaced at some point.

from memory, they look like mine

Definitely don't look like OEM. Just the size make them seem like they are replacements (maybe these?) of the '49 state' variety - much less substrate than stock.

Btw, 2Bennet has replacement downpipe a with cats that look exactly like what you have.

on the 2.8 the downpipe and cat are all one assembly connected by the flex joint.

He has 2.7T, but they are all one unit, for all engines.