Donald asked: " Is TREK the only company to make investment-cast (IC) head-tubes?"
I don't believe that Trek did use investment cast head tubes although they did use IC bottom brackets, rear dropouts, and seat lugs).
I think Trek was using Nikko Sangyo or Fukurotani one-piece head tube/lugs which are pressed steel. I am sure Trek had their name embossed during the forming operation.
About their "fruit of bulge-forming and cold working technics", Nikko claimed their one-piece design "eliminates assembly and brazing required by 3-piece heads, eliminates re-braze so frequently necessary to fill gaps, retains the clean sharp fetures of lug design by eliminating the clean up grinding operation, and can reduce cost by as much as 50 percent compared with a 3-piece head.
Cheers,
Hugh Enox
La Honda, California USA