Seems there are two at least two major versions of the 2700, or there was just a frame switchover at a certain year.
I have seen pics showing a Tange hi-ten steel frame sticker, and ones showing Tange Chamipion (with no number) I have a 2700 in my possession that has the Champion sticker, which I believe says cro-moly on it in tiny letters ( I would have to check when I get home).

The Hi Ten version 1 all seem to have flat seat stay tops. The only pics I can find that clearly have the same type of sticker as mine (version 2) indicating a Champion tubeset are all gold/tan in color, and have scooped seat stay tops. Mine is silver, and has bar end shifters, and braze on cable guides, but otherwise looks identical to the gold version 2 below. None of the pics below show GT deluxe on the top tube, mine does not either.

I am sorry to steal any pics, I will remove the links if the owner objects.

Version 1 :
http://www.ece.ubc.ca/~gillies/sekai..._about79_4.jpg
http://www.ece.ubc.ca/~gillies/sekai..._gt_deluxe.jpg

Version 2
http://www.ece.ubc.ca/~gillies/sekai...eftSeatlug.JPG

I will have to find S/N and date codes, but I am seriously wondering if there were two different tubesets at the same time for the 2700, or if there was a just a wholesale move away from the hi ten tubes in 1979-1980?