The lever is used to operate the cream whipper, i.e. to dispense the whipped cream. On older whippers, the lever is made of metal and is paired with the metal dispensing piston.
The new design is incompatible with the old one, and has the lever made of plastic (part number A65). It uses a different piston, also made of plastic.
The spring, which is responsible for the return of the lever is riveted to the lever. The lever controls the flow of the whipped cream by moving the dispensing piston.
Unfortunately, the metal lever with part number A41 cannot be bought as a new part. If you need this part, the best solution is to buy an old (either defective or incomplete) cream whipper head and use its parts as a replacement. It is worth paying attention to that the spring should be fixed to the lever and preferably not rusty.