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1987 cooling system seems to be both open and closed


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Hi everyone!
I have a 1987 xj limited that I’ve owned for about a year. For the most part she’s stock, but there are a few head scratchers. My current puzzle is the cooling system (I’m a total newbie so please bear with me). I’m in the process of flushing the system after seeing some rusty sludge (thankfully not oil!) in the overflow reservoir and discovered something odd

The overflow reservoir is definitely stock and for the original closed system. The cap on the tank is not vented, again leading me to believe that this is a closed system. However, I also have a newish radiator with a radiator cap. That should only exist in an open system, right?

My current theory is someone converted it to an open system but didn’t know enough to replace the reservoir (or replace the cap with a vented one). Has anyone seen this before?

Any thoughts are greatly appreciated!
Technically you still have a closed system since the tank is not vented even though you have the radiator cap (I Believe).
I just did this (Mainly because of rad price) before a trip to Moab in Oct and havent had any issues after 6 days of wheeling.

But now that Im thinking about it, I dont recall what I did with the small hose that comes off at the rad cap.