Clean out cap spins does not come out

[TJ1]

The cast iron(?) sewer clean out port is located in the garage. The brass cap turns freely but does not come out. I can't see any threads on the cap, and it feels there is something inside that catches. Maybe what is left of the thread?

Should I cut off the square then pie cut the cap off? I just don't know if this is a standard cap and what weird structure might be inside.

Thanks!

 

[TJ1]

Update: I sawed off the square, then proceeded to pie cut the brass plug. The ring that turns with the plug has a non standard size opening. I had to saw off the threaded portion to expose the cup, which is somehow secured to the pipe. Then a 2" rubber test plug is inserted to the cup.