Diff cup compatability

[lincpimp]

So I was wondering which 1/8th scale diff cups are interchangeable. I know that ofna, hot bodies, and kyosho use the same cup.

HPI savage cups have a larger area for the bearing to sit on, but the internals look to be identical, are they?

Any other cups that will work? And internals? Just looking at all of my options to put together a kit for a center diff for the new emaxx and erevo. I want to offer the best choices in cups and gears if I sell it as a kit, just to give people options.