I seem to remember there was a service hole on some drum brakes ,? Used to like a screwdriver through to adjust
Yes, there was a hole in the back plate.
Handy if you didn't want to remove the drum.
From a garage service point of view, the procedure was put the car on a wheel free hoist, remove the drum, check the shoes for wear, scuff the shoes with emery cloth, blow the dust everywhere, then click the self adjuster.
It was also easy to pop the drum back on again, check for free spinning, and give the adjuster another click if it would take it.
Incidentally, most drums had a small countersunk set screw to hold them in place.
I could never work out why, given the drum is also held in place by the wheel and its four studs and nuts.
Or three bolts and a locator pin if it was a Renault, but that's the French for you.