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."
Bikes are cool, even the dumb ones.