Brake drum clearance

It could just be a little rust or something between the edge of the drum and the backing plate.
If you're really curious you'll have to take the drum off to look for contact marks.

Some good tips on brake service in the master tech series.
Master Technician Service Conference - Chrysler's Training for Mechanics
Scroll down to '60s.

Just be careful when looking the ones from '69 up. In 1969 adjuster hardware and shoes changed. So those won't look the same and (work in the opposite direction).