I know that Ishiwata's triple-butted cro-mo came in both seamed and seamless versions; my Bridgestone RB-2 features the seamed tubing, whereas the RB-1 featured the seamless variety. The thicknesses were different, I think, but I'm not sure about that.
My early 80s Fuji Del Rey with its quad-butted VaLite tubing . . . I wonder if it's seamed. Wouldn't surprise me . . . I'm pretty sure the seamed tubing is less expensive.
I'd guess that lots of double-butted tubing can be found in a seamed version; maybe Richard can let us know when he gets back from that 50-mile ride! Or maybe one of the other framebuilders on the list? . . .
nath dresser
spring green, wi