It looks like a standard stainless grub screw with the usually-pointed end flattened off. They're readily available in several lengths.
Rocnel have been quite sensible and generous in this design, which allows the use of third-party handles. They could have produced a solid proprietary handle, with a turned down threaded end where they currently use the retaining grub screw.
Of course, with such a handle, it wouldn't be long before some bright spark would work out that a grub screw could be used instead.