Brake Shoes
flawed engineering in my opinion.
Thats a new one for me. I know the groove was meant for venting/cooling the asbestos linings and failing adhesive at extreme temps may have played a role but still,I have never seen that on a 10 inch a body drum and I have used NOS Mopar drums recently. I can't recall seeing it on any new drum. I think it would be pretty hard to turn leaving the raise in the drum. Every machine I have seen turns the drum equally all the way across. If memory serves me correct I recall seeing some brake shoes with a groove near the center of the lining. If I am recalling right this would wear the pattern in the drum over time.
flawed engineering in my opinion.