As I recall, the Tange tubing was seamed and the Reynolds is not.
The manufacturing process for cold drawn seamless tubing is pretty amazing. Starts out as a solid piece of steel, which is pierced and drawn into a tube. The Tange tubing started as a flat piece of sheet steel, was rolled into a tube and the seam was welded.
If I recall correctly, the Tange people said because of all the cold working done to the Tange tubes in the butting process, etc., the seam basically disappeared and their tubes performed as well as 531.
I always wanted to take the stickers off of a couple of Treks, one 531 and one Tange bike and do a blind performance and feel test. Never got around to it, but my bet was it would be hard to tell them apart.
I think the Trek people used the Tange tubes to squeeze Reynolds on the costs or even eliminate 531 bikes. That never happened in part because they could not convince the buying public the Tange tubing was just as good. Buyers still wanted the Reynolds 531 Aluminum (sic) bikes.
Later Reynolds introduced the 501 tube set, which I think was seamed tubing.
Columbia Missouri USA