found this in the archives, hope it helps.
"Adam, I have just returned from holiday and have taken a look at a Sekai catalog from the late 1970s. The seat tube and head tube decals match, so the era appears to be correct. Based on the stamped dropouts and single braze-on for the rear derailleur cable stop, the frame is limited to three models, the 1000, 500 & 400. These are the 3 bottom models in the lightweight line-up.
Fortunately, the Sekai specs are very complete and we can futher segrate the models based on seat tube diameter. The 1000 uses Tange, hi-tensile, single butted tubing and employed a 26.2mm seat post. The 500 and 400 use Tange, high carbon, plain gauge tubing and used a 26.0mm seatpost. 0.2mm is not much of a difference, but if you're lucky enough to have the original post, it may be marked with the size and help to distinguish which model you have."
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