Don't know for sure, but different hubs can take slightly different centrelock lockrings, depending on axle standard. Think they generally come with the hubs. Every time I've needed a set someone has come up trumps on this forum.
Yeah I realised that when I came to order some. That being said, the limiting factor in all this is the size of the splined interface not the axle size it would be easy to make one to fit any axle size, the 15 & 20mm one appear to be the same too.
Spent £130 odd on a pair of sram discs a couple of weeks ago just come to fit them and there weren’t any lock rings, is that normal ?