Clearly that is quite different. I suspect that RFI may have been one
of the reasons they went to the much lower frequencies...

FYI, my coupler has a 4-pin Amphelnol connector for the radio input
and a 9-pin Amphenol connector for connection to the DG and autopilot
amplifier. In my case, the coupler could not be eliminated from the
system by simply moving connectors around.

Too bad. With the Century (aka Autocontrol) units the basic "computer"
plugs in to the AH and you have wing leveler capability. If you have
the right DG (i.e. with the heading bug) then a second cable plugs into
the DG and you now have heading control as well. [Hence the separate
ROLL EXCITATION and HDG EXCITATION.] When they added the little NAV
Coupler unit, all that was needed there was to unplug the computer's DG
cable from the DG and plug it into the NAV Coupler. The NAV coupler
then had a cable that went out to the DG.

With the NAV Coupler in HDG mode, it was essentially then just a
pass-through. The DG signals basically went through the NAV Coupler as
if it was nothing but an extension cord. [Except, as another poster
mentioned, the NAV Coupler still generated a new version of the
EXCITATION signal.]

With the NAV coupler in tracking mode, it simply took the DG offset,
played the same "in/out of phase" game that the computer normally would
to provide an offset, added in the offset from whatever L-R signal it
was tracking, chopped it back up, and sent the new sine wave back to
the computer. The computer never knew it was tracking anything but the
DG heading bug. [BTW, all the nav coupler tracking modes are just
variations on a theme. The LOC mode (Localizer) just has a different
divider resistor on the L-R so the sensitivity is changed. The OMNI
vs. NAV mode just adds a capacitor to slow up it's ability to respond
to quick changes in the L-R signal. And the LOC Backcourse? Yup...
the switch just switches the L-R input wires so that it responds
backwards.]

[I'd be happy to learn all you're willing to teach me about these
things. If you have any schematics or theory of operations on the
early Autocontrol / Altimatic
I/II amplifiers and/or on the early radio couplers, I'd really
appreciate getting a copy. ]

A while back I scanned in the schematics for the Autocontrol computers
and the NAV coupler. I don't know if they are still around or not
(really poor quality originals). I'll try to see if I can find them.