With you on the Japanese and GI rods, but the cases were never boxwood. Box is among the hardest of woods and very dense. It is the origin of the word box, but originated from its use for tough, strong, hard, and heavy turned containers for tiny religious icons and possibly very early coins.
The wood used in the rod cases I have seen have been aspen, paulownia, ash (tamo), or soft maple. I have heard of beech being used but have not seen it.
