Mainly to allow adding the Arca-Swiss mount to the foot without extra size/weight of a plate. With the 100-400 II it was a simple swap of the foot but with the 100-500 it is either add a plate or swap the entire collar/foot if that becomes available. I should state this is not necessarily for me but for work as I'm fine with a plate. Have a plate on my RF70-200 but have yet to use the 100-500 other than handheld. My user base will need to have the 100-500 on a tripod.I’m curious, why a replacement collar? AFAIK, they’ve only done collars for a couple of Nikon lenses with poorly designed collars. I’ve been happy with my Canon lens collars. In addition to the included ones, I purchased the Canon collars for the 70-300L and the 100L macro. I do use the RRS replacement foot for my 600/4 II, but the collar itself is fine.
The RF collars do lack the detents at 90° intervals, is that the issue? I haven’t used the lenses on a tripod yet, may never do so with the 70-200/2.8, and the 100-500 might be used very rarely on a monopod. My main use for the collar is as an attachment point for my BlackRapid strap (connected to the lens plate with a Kirk 1” clamp LocTite’d to the strap lug).