[Richbot]

My unscientific observation was the RB pads included with the rear rotors were the same thickness both friction material and backing plate as new OEM pads I also had, but did not have the polymer noise-reduction backing on them. I bet most aftermarket pads don't have that backing. The OEM pads fit, but just barely (like tight enough that they would have dragged) so I used the RB's, which are a tame street compound, for now.

as for why, can't say, but I'd bet they didn't want to make a new casting just to make a 24mm friction ring and already had a 350x26mm ring for some other apps (looks like they use this dimension ring in some other BMW and merc apps, maybe the Ford Raptor?) based on their website obviously.

It fits fine now. I did not make any changes to the passenger side bracket, the driver's side is filed down because I was trying to make it work, but in the end the hats they originally sent me were off by about 2.5-3mm and no amount of filing was going to fix it. They (I assume) have fixed the drawings/design control issue, because the set I have on the car now fit fine first try.

As for days grinding brake material - belt sander and a brick? Neighbor kid?

EDIT: Warren tells me I got the one and only set of assembled rotors that shipped like this, due to some internal design changes they made to their BMW lineup which happened to be in progress when mine shipped. So, fluke, hopefully. And like I said, the set I just got back from them fit first try no problem. The hats on my car now have a different part number from those RB shipped originally.