Not sure we're talking about the same hardware item. The reefing claw I spoke of has a circular shape like the boom and an opening to slide it over the boom end. Wheels on top allow the boom to be rolled or unrolled and an attachment point is provided on the bottom for the mainsheet.
If I'd had that gadget I would have loved it on a Filip Viper 15 we owned. We added a slab reef but that roller would have allowed a lot more flexibility. We could have custom reefed it to the specific wind of the day.
Unable to cut and paste the link I found.System won't allow it
A boom vang is typically a tackle like the mainsheet.