1. There is no bushing for these?
2. You would not use a grounding bushing but a grounding locknut as shown?
3. You would need to remove regular locknut it comes with and buy a grounding locknut separately?
It appears the "HUB" series does not have a grounding locknut whereas the STG series does.
SevenDelta's chart should help you decide if a grounding locknut is required. If not refer to 250.92 & 250.97