variant binding

Does anyone here know the source of the assertion that copies of The
Collector by John Fowles which are bound in charcoal-black papered
boards (as opposed to rust colour) are a trial binding. A couple have
been offered for sale, so they exist, but are they trial bindings as
opposed to, say, variant bindings? If the former then it suggests
precedence, and therefore justifies a higher price tag.

F.B.