Never heard of it but this is what Wikipedia has to say:
Japan drier is a common lay term and generic product name for any oil drying agent that can be mixed with drying oils such as boiled linseed oil and alkyd resin paints to speed up "drying". The name refers to "japanning", a term for the use of drying oils as an imitation or substitution for urushiol based Japanese lacquer.
Dan, sounds like you are right on the money.