"White Belly" was a term coined by the Asian LS3/5A affictionados. It was not an official term by any means. These appeared in LATE 80's 15ohm units.
Those Kef drivers that did not pass BBC specs were "repaired" by Kef using this white adhesive binding the cone and the dust cap. KEF added an amount of this glue to the dust cap area
during the last years of production of the 8 ohm unit, probably to stabilize it and possibly also to add some mass. Once they passed certification, they were now fit to serve as LS3/5A drivers and appeared with the distinctive white ring around the cone. Some people say they should have been called "white navel" instead of "white belly"
As to price, apart from performance that is attributed to it by its die-hard collectors, I think the novelty and the story behind it largely drives the value as well.