The #50 torx tip is needed to stop the piston shaft from rotating in the shock housing while undoing & tightening up the the retaining nut.
As I stated earlier some aftermarket shocks no longer make use of a torx design for the socket interface at the top end of the piston shaft.
Spinning the piston shaft with an impact gun is not desirable. Doing so with the outgoing shock is alright but no so with the new shock.