Paraphrased exchange from BikeForums:
Question: I have an unidentified year-probably 1985-Lotus Supreme. It has fork and rear Campagnolo dropouts and cable guides. I’ve been fiddling with it (it was setup as a horrible fixed gear when i picked it up) and noticed the steer tube says “TANGE” “MADE IN JAPAN” and has the Columbus dove logo stamped on it also. Aren’t these two different types of tubing? Why would they both appear on the same bike/fork? The [bike’s] main tubes are and fork blades are Columbus as they have the original stickers.
Answer: That’s a new one! Are there five rifling splines as you’d expect with a Columbus steerer? If so, I’d guess that the “TANGE” is just to note who made the fork, not who made the steerer or the blades. Take the front wheel off, and peer into the open steerer tube of the fork (from the bottom). It should have some ridges in it; it’ll look like the gun barrel out of a James Bond flick. Tange and Ishiwata would build sub-assemblies out of whatever tubing the customer specified. We (Trek) had Reynolds 531 forks and rear triangles built by Ishiwata for use on the 600 series models. The sub-assemblies were brazed in Japan and shipped unpainted to Waterloo, WI, where they were mated with frames, painted, and assembled.
Conclusion: So the “TANGE” “MADE IN JAPAN” is just a reference to who assembled the fork/frame and not the material used? Makes sense-it is a Japanese bike after all.