Given this short paragraph about the L-21 submarine, what was an example of an improvement made compared to earlier versions?

The World War II Soviet submarine L-21 belonged to the L-class or Leninets class of minelayer submarines. She was part of the last series (Group 4) of her class, having some improvements including more torpedo tubes. The commander during the war was Sergey S. Mogilevskiy.

The L-21 had more torpedo tubes.