The original setscrew had a square head with a hole drilled through it for securing it with stainless seizing wire to keep it from backing out.
When dealing with a sheared key, be sure to check that the new key fits in both the shaft and the hub with no side-to-side play. If either keyway is worn, all the load will be transmitted to the setscrew on every fwd-reverse cycle. This will quickly loosten the setscrew, and shear the new key. Unfortunately, the only fix for a worn keyway is to replace the worn part. This is not bad if its the coupler, but an expensive pain if its the shaft.
If you're replacing the coupler, why not consider one of our sponsor's split-hub couplers?
https://moyermarine.com/product/dire...orev_14-1_542/
This will meke future removal much easier. They come in 3/4", 7/8", and 1" sizes.