Relative to the 555=Tange theory, and seaming: Tange Infinity tubing was (I read somewhere in addition to VeloBase) seamed, double butted, produced in the mid-80s. I have a Nishiki frame that's Infinity -- happens to be the same "Prestige" model with chromed seatstay caps that I believe is pictured on the third link above -- and it's not a heavyweight either; compares favorably to my Raleigh built of 531C -- frame a bit heavier than the Raleigh, but fork lighter. Fork is marked Tange.5.C on the steerer tube, which does not have a seam that I can see (on the interior). The head tube is definitely seamed, but the seat tube -- I'm not sure at all -- there's a faint line running down the back side of it, that looks like a rust stain more than anything (right below the seatpost slot, too); but I can't feel a seam and if there's one there, it was pretty much erased by mandrelling/drawing the tube. 26.6 mm seatpost, by the way. I've always thought that it was funny that Nishiki's "Prestige" logo is the same typeface used for Tange Prestige tubing, but the tubing decal says Infinity.

The Falcon frame I have that has seamed tubing -- the seam is quite evident all the way down the seat tube.

I know -- too much information -- but there it is.