Question about 7 1/4" rear end differences

At one time the 7 1/4 with 9'' brakes had drums with a Bell design. Not sure of years.
Found that out when looking for the 10x1 3/4'' drums thinking they only had the Bell design.
What's the purpose of the Bell design. Did brake drum sizes change from 68 to 71? Bolt pattern should be the same right?