Circulators are often the most expensive part in a complex RF circuit, as the design, manufacturing, and speciality of the device.

Shown in the image here, you might be able to see what look like cut marks or scrapes on the PCB dielectric. These cuts are where extra material was likely left in the design for manual tuning by careful removal of copper material on the etched PCB in order to achieve the final desired performance of the circulator circuit.

This particular conductive RF circuit uses a PCB with a highly specific dielectric insulator material and multiple connector ports in order to create what is known as a circulator with three ports, or an isolator with two ports (in which the third port is terminated at the matched impedance of the circuit).

Two incredibly strong permanent magnets placed on both sides of the PCB create a magnetic field which forces the electric field to behave in tune to it.

Reopening this since I still haven't found the exact item and my OCD isn't letting go of it.

Think of it as one of these retaining nut, but only the star retaining washer and the press-fit insert that was threaded to match the nut. There has to be a part out there right?

The retaining portion sits on a captive screw where the threads were turned down to leave space for a washer. Maybe the washer was a custom item, unsure still.