Originally Posted by mainstreetexile

Unfortunately, I don't have mine anymore. I was short on storage space at the time and I regrettably got rid of that frameset (and a very nice Koga Miyata Terra Liner Carbon!!).

Mine was a 1974 champagne colored frameset, and I found the old text for my for sale ad, where I noted that mine took a 26.6 seatpost:

From what I've read, these were Nishiki's top end frames and they were handmade at a separate factory/location. They were trying to make a Japanese frame that would rival hand-built british or italian racing frames. Tubing is marked as "Cro-Mo 0.8-0.5-0.8 double butted", but from reading it sounds like it may use Tange Champion 1 double butted tubes, with suntour fork ends and vertical suntour dropouts. Some nice filing / cutouts / details in the lugs, fork crown, brake bridge, and bottom bracket shell. ... Takes a 26.6 seatpost and a standard bottom bracket (there was a bottom bracket in there when I got it, so the threads are good).

I have some other Nishiki ONP info saved from that time, a 1978 ad and magazine review of one. The ad had frame measurements and a note that stated:

* Please install the seat pillar of 27.0 or 26.8 (620mm or 640mm)

So maybe the seatpost size or tubing widths changed depending on the year or frame size?