First Mixte for me as well
 

I saw this Nishiki Olympic on FB Marketplace yesterday morning for $20 and had it in the back of my truck by noon. It's filthy but the frame, paint and decals are in excellent shape. The only thing identifying the frame material is a label saying "Special 150 Tubing". Anybody know what that means in Nishiki speak? I cleaned a few areas and the gold paint shines up nicely. I'm in the process of dismantling it for a proper clean and rebuild as a parts bin flat bar 7 speed cruiser/hybrid type thing.

https://cimg6.ibsrv.net/gimg/bikefor...c68c524c1.jpeg
At 19 inches the frame borders on being too small for me but perfect for my wife.
https://cimg7.ibsrv.net/gimg/bikefor...1c4a77642.jpeg
Some nice lugs on the head tube as well as the rest of the frame. That scratch on the last "i" is the only blemish on any of the decals.

Probably the modern Japanese equivalent of Tange #5 - basic cr-moly that gets the job done. I think a bit of metallurgical ingenuity had to go into those mixte "top tube" bars though.
I have seen very little on the subject, and have read even less.

The Nishiki bikes I've seen new were pretty nice bikes. The brand has a bit of a cult following around here.
Good luck with the build!

-D.S.