Surprisingly the entire winch spool is secured by that one screw in the bottom of the socket well. The design of the winch proves the screw sufficient, only so long as you don't compromise the fair leads.
The new GRCS training video "Safety First" gives insight to all of that.
The GRCS is the epitome of the finest arborist capstan winch and braking system rolled into one. The best design and engineering proven through real experience, But as with everything in this business a fool can prove anything otherwise.