I've seen this on several British shotguns. What does it mean?

A bunch of bend. Not as much as a crossover stock (for someone who's right handed but left eye dominant, or vice versa) but significantly more than the normal 1/4" or so of cast.

Central vision cast is a mystery to me. It may be usable for open choked guns on big slow targets, but obviously not the ticket for precision shooting. Good shooting at difficult targets demands that one eye or the other look over the middle of the shotgun at the target. "Central vision" would imply that you could be looking over the gun with either eye, depending on which one is dominant at a given time. To me, this means I would either be almost missing on the right side of the target or almost missing on the left side of the target. With tighter chokes, I may be missing on the right or the left.

these guns were designed for shooters with "no" dominant eye....as in both eyes of equal strength. This is why very, very few people require one and why they wont work for you eightbore, or me for that matter. Special guns for special requirements.
Steve

Probably much easier to do something to shift the dominance to the right eye. If they are truly balanced, a darker tint lense in the left eye should do the trick. You don't want to eliminate the vision from the left eye, just lower its dominance. You might also be able to use a lense that is just slightly off focus for the left eye, which should shift dominance. I know most of us have seen the Scotch tape trick on the left lense, but I've found it very distracting.

Using different color lenses is a great idea, Chuck. Wish I had thought of it.