[Rover]

The 400mm/600mm look as if they just attached an adapter actually.

Which is prolly exactly what they did. I theorized elsewhere that this is done with two subtle (and insidious) purposes: not only to sell basically the same lens to the users again, but also to prevent the adaptation to the different ML mount bodies, as now there are (smart) adapters from EF to just about any ML mount but none whatsoever from RF to anything.

Gah, my tinfoil hat's gone missing again!?