Need to ID drain piping under sink

Below kitchen sink and yes that trap at the disposal is off and has DIY written on it but my concern is the pipe which comes out of the wall on the right side and slants down and back into the wall with a cleanout.

Any ideas why that would be there ?
Sure does not look like an island vent however from the roof the vent is about 20 feet away as far as I can tell.

This was a 113 year old loft conversion of an old movie studio.

Its just a improper add by home owner or in qualified contractor, looks like a company stuck it in after repeated calls to clean secondary stack.lools like a steel galvanized tee to access a repeat clog installed by a drain cleaning company, its effective to get the tectnition out but doesn’t solve the initial problem because the secondary doesnt work or drain like it should do to breakage at the floor level or a semi perminate obstruction.A video scope will identify thr problem but it is leaning to breakage or improper fall which causes future back ups and many repeat service calls.

So you do not feel it is a bad island vent attempt then.

What do you mean by secondary stack ?
There was no vent right in that area when I went to the roof (4 stories)

Breakage ?

Sorry but I am not quite following you.
Appreciate the help as it is in short supply so far however.

It appears to be a “non-specific Polish plumbing modification”. Common in the Chicago area.

Must be, judging from the lack of answers.
Perhaps Robert you are on to something.
No gurgling but the dishwasher was shut off and makes me think there is no island vent which may be needed here.
That could be a reason why they did the cleanout though no sure why it has 2 ends there.

Bob… as you well know… converting some of those old structures to lofts can be challenging, and sometimes things are not as conventional as we would like. My WAG would be for improved venting, as you mentioned you did not see any additional stacks in that area penetrating the roof. Could be the original stack needed repairs or relocation, and it was more practical to install a secondary stack than to punch through to the roof. JMO.

Please explain what you mean by secondary stack Jeff.
I saw no roof vent in the immediate area.
Are you referring to island vent ?

Not specifically. I wasn’t even considering an island, but it may be related.

Basically… Vent Stacks and Waste Stacks…

You said this structure is 100+ years old. Visualize what the building looked like before. What kind of plumbing system did an old theatre have back then? Not much! Now how do you think the system would have been modified to handle all the lofts that were developed. You know they re-used as much of the old original stack as possible, but in no way would it come close to handling the new demand. The original stack may have been converted to “vent only” (no leaking) and all waste was installed new (without a stack penetrating the roof). I guess it would be very similar to an island vent system, just in the walls and not an island.

Sorry if that doesn’t make sense. I’m heading out now, but will check in later and try to post a graphic, if I can locate one, to help explain it better.

Not only that but the building had 2 stories instead of 4 at one time.