I just got back to the area, and have made some discoveries. The Local Normal DX Switch seems to also function on AM BC as an RF Gain Control. When in the Local Position, WJR reads ~47 dB and WWJ ~36dB. When in the DX Position, WJR reads 44 dB and WWJ reads 51. If you add 40 dB to the reading in the Local Position, I'd say that it reflects reality. The Summer Day signal is probably in the vicinity of 6.3 mV/m and the WJR signal is in the vicinity of 22 mV/m. Beyond that, we'd have to construct a fudge factor curve with the FIM-41 next to the PL-880. The DX position seems to have the AGC Stiffness, and even with a fudge factor, results would be complicated to analyze.
To those of you who think that "fudge factor" sounds like fudging the data, it's really a well known and accepted technique of making sense of data, not altering the results. Calibration of a FI meter is like an internal fudge factor generator which compensates for the overall receiver gain at different frequencies. You also may notice that the d'Arsonval movement markings on an FI meter are NOT LINEAR either, and is part of the calibration process. In Physics, an ideal spring is linear, but a real spring is not completely linear. The force of the spring balances the magnetic force from the coils in the d'Arsonval meter.
If you took Chemistry, Physical Chemistry, Analytical Chemistry, or even Medical Technology classes, you probably encountered a B & L Spec 20, which also illustrates a lot of the same principles. A Spec 20 is essentially an analog Spectrum Analyzer for Visible Light, Near IR, and Near UV frequencies/wavelengths. You always have a "Control" reading, which was also necessary in old FI meters, and apparently has been simplified with new FI meters. The Control has to be repeated at each wavelength to compensate for the source amplitude and light detector sensitivity at each wavelength. Spec 20s are still widely used. They have probably also been improved over the years, though like FI meters, the old equipment is still widely used.
https://en.wikipedia.org/wiki/Spectronic_20