One visitor to PMA who has contacted me remains sure that the larger lens will be a 400mm f/2.8
because its size is so similar to the Canon of that specification.
Because of the length of the lens hood, which reveals no front glass in
other views of the lens, it is impossible to tell.
The grab handle is
identically positioned to the 600mm f/4 and although the rear
barrel sections are stepped in different thicknesses, their overall
length and scale relative to the filter (dummy) slot and rear mount,
and the control buttons, all compare directly to the old 600mm f/4.
However, a 400mm f/2.8 and 600mm f/4 share the same approximate front
group size. It just looks rather long for a 400mm given advanced in
Text and Imges from PhotoClubAplha.com
Images from WATCH-Japan