I could be wrong, but I believe the "Imported Briar" stamp widely used in the U.S. was a reflection of the scarcity of briar during World War II, hence is mostly used in the forties and ongoing even into today in some cases. It was a way of saying, this pipe isn't made of some substitute wood; it is really briar. I'm not sure the U.S. experienced the shortage before it entered the war, possibly coasting on inventories already in hand. For a while, Mountain Laurel was used as a substitute, which is quite good for the purpose, but I know only one carver who still works in that wood. And yes, I believe in the 1950's, Kaywoodie was the major premium pipe in the U.S. costing more than the Dunhills.