Yes, the adjuster for the little parking brake drum built into the
rear disc brake rotor.  I looked more closely at the backing plate and
it has what looked (only a little) bit like the nickel-sized knockouts
one sees on electrical outlet boxes, which are quite easy to remove by
poking with a screwdriver.  But they wouldn't budge when I beat on
them with an admittedly blunt chisel and a decent-sized hammer.
Started denting in the backing plate.  I eventually got the rotor/drum
off each side by using a big gear puller on it.  One side flew off and
fell on the floor without damage; the other side ripped off the
mounting hardware inside and hung up the rotor/drum from coming off
more than about 3/4" as the brake shoes cocked sideways as I tried to
remove the rotor/drum.  But it came off far enough that I was able to
reach in with a screwdriver and back off the adjuster enough to then
get the rotor off.  The gear puller action had also ripped the linings
from the shoes.

As I'd suspected, there was quite a rust buildup in the area not
contacted by the shoes (it was now a smaller diameter than the shoes),
making it quite a challenge to get the rotor/drum off.  What was Ford
thinking by closing off the holes?  The factory service manual only
says to reach in through the holes and back off the adjuster.  !