The "Lock" of the final season is Tommy’s own psyche. For five seasons, Tommy used trauma as a fuel source, but by 1933, the fuel has become a poison. Haunted by the ghost of Polly Gray—the moral "anchor" of the family—Tommy finds himself physically and spiritually confined.
The "Key" to Tommy’s survival has always been his ability to "change the skin." In the series finale, we see the ultimate subversion of the show’s central motif. For years, the Peaky Blinders used violence to unlock doors to high society. In the end, Tommy realizes that the only way to be truly free is to let the "Thomas Shelby" the world knows die. "Peaky Blinders" Lock and Key(2022)
If you are looking for a deep dive into the 2022 "end of an era," here is a feature on the themes of entrapment and liberation that defined the final chapter. The "Lock" of the final season is Tommy’s own psyche