Help identify my bike frame Please help me identify a mystery bike that I purchased at a garage sale and have grown to love.

I am not sure what is original to the frame but the bike has a 56cm frame, Shimano SFR rear dropout, a sticker on it that says 'Champion NO.2 Tange Industries', 26.8mm seatpost, serial number on lower non drive side seat tube that I believe says 'S0S5140'. It has braze-ons for one bottle cage on down tube and on top of bottom bracket a cable bridge.

The components on it are first generation Shimano Dura-Ace cranks, bottom bracket and brakes. Suntour Cyclone front and rear derailleur with Shimano 600 Arabesque shifters, 3TTT stem, Tange headset, KKT pedals, SR seat post Concor S. Marco saddle. Campagnolo hubs with Fiamme tubular rims.

It fits me well and rides great, it just would be great to know who made it.

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That's definitely a Japanese frame and my guess is the wheels were an upgrade somewhere along the line.  The mix of Shimano and Suntour also is unlikely to be how the bike was sold. My guess is the Cyclone derailleurs were an upgrade, but since it's friction shifting it all plays fine together.

A web site with a lot of experts is the "Vintage Cream" Facebook group.  I would suggest adding more closeup pictures of the lugs, fork and bottom bracket. Tange #2 and Shimano forged dropouts are clear indicators of a very well made frame.

Vintage Cream FB Group

Good luck in your search,
Greg