It's Tange Champion #2. The earlier versions use the peerless high carbon steel tubing double butted. I have owned two of the 1977 Sekai 2500 bicycles. Ask me anything about the bike and I will tell you whatever you want to know! The 1977 price was $285 + tax, this looks like 1979-81.

Truthfully all that matters when it comes to butted steel tubing is the wall thicknesses and the lengths of the tapers. Its impossible to tell 9/6/9 Reynolds from 9/6/9 Tange tubing ( and this is the touring gauge that this bicycle is probably made of )...

https://people.ece.ubc.ca/~gillies/s..._gt_deluxe.jpg