Guess what I found.
I purchased a N.O.S. axle from Builder 77 about 5 days before he was called home to help wathc over the rest of us.
I installed the axle but used my existing brake drums as I did not feel like cleaning the red finish off that day while I was in such a hurry.
Guess where the slot is (are), not in the backing plate but in the drum.
Here is a picture of the slot outlined in dust.
I also checked my O'Reilly's replacement drums and the also have the slots as well.
Punch this little sucker out with a 1/4 punch and you have your access hole (slot) for backing off that little wheel.
You need two long slender tools and a light to back of the adjustment wheel.
Jimm1009
In going back to your picture of the drum, I think that I see the plug at your 7 o'clock position but
You should be able to find it pretty fast with a small steel brush or some sandpaper.