Ken Scharf wrote:
as I rotated it in my hand I could feel something
inside shifting back and forth.

The inside has the resonating elements "suspended" in foam. The foam
does decay, but usually in place.

Check the resistance on each side of the filter, it should be something
like 50K ohms. If either one is open circuit, the tiny wires that
connect to the coils inside have probably broken. Check the R-390A
sites, some have taken off the end of the filter can and shown pictures
of the inside, and some have even reconnected broken wires and at least
thought about replacing the decaying foam.

The 4kc filter is the hardest to find but not impossible (either demand
or most used/most quickly worn out), the 2kc filter ought to be easier.

Tim.