For my 8 Phase II rotary table, the 6 chuck mounts to an 8 back plate giving a 1 lip all the way around to make it easy to bolt the chuck/plate assembly to the rotary table T-slots. There is also a round alignment disk (roughly 1.25D that fits snugly in a hole in the center of the RT directly above the MT socket and a matching hole in the center of the mounting plate. Presumably the intent of this alignment disk is to allow you align the chuck to the plate and RT once, and subsequently all you need to do is re-bolt down plate to RT next time. However, in practice I tossed the alignment disk aside, as there is no avoiding using a DTI to align the chuck to the RT every time it is mounted since tightening the bolts to the RT impact the alignment at the nose of the chuck by quite a bit. When I got an indexer and a dividing head, I made sure they could accept 5C collets and had the same spindle nose as my lathe so I can move chucks and collets between the lathe, indexer or dividing head without any of the realignment hassles.
Robin