This comes with a modified bcg, it will only work with this setup, not a standard AR. The buffer and springs are included. This system replaces the standard bcg, receiver extension (buffer tube), buffer spring, buffer, and stock/brace.
You may have confused the 2.5 and 4 inch versions.
The 2.5 inch kit included the modified BCG required to use this brace. The 4 inch versions use a standard BCG which is why there is not one in the kit.