My best guess is that it's just the last element of the objective unit. I think that the lens element in front of it (closer to the objective end of the bin) is the focusing element because it looks like it is on a movable structure. So I suppose one could say that the B1's use a four element objective (with the 3rd movable element for focusing).
http://www.meopta-history.com/?id=176
Yes, I think that's it. The focusing element is forward, instead of
moving the ocular. I got to visit an optical engineer and do some
ray tracing. Moving a field/focuser behind the objective has a very potent
effect on the focal point (so it makes for easy smooth focusing). It also
provides another effective gate for glare.
There is a little downside, though: because of the way the ray angles are
bent in more than if they were from the objective, the apparent F/ratio
as seen from the ocular is lower, so the depth of field (range over which
you can hold a certain resolution) is reduced.
This was my interest in that lens: it occurs in many binoculars now,
and I had seen the effect at home and in the shops, where there
is almost a "moving focal stage" until you get out past 100-200 feet.
Just something to consider when you're picking up a pair for a given
context. It can be a nice "featuring" effect, especially of you
are taking photos through a barrel. For dynamic 3D action (like
nearby bluejay mobs) you might want more depth or to be farther away.
It does make for very smooth rugged focusing, of course.
The design doesn't have to dangle two hefty eyepieces in a harness:
they are mounted, and the moving element is braced by the central body.
