Serial number indicates a 1977 Mikki built frame. The two major brands built by Mikki during this period were Centurion and Sekai. The cinch slot on the seat lug looks pinched at the top by about a millimtere, so the proper post is probably about 26.8mm which would be correct for a Tange tubeset. To the best of my knowledge, Sekai did not use a riveted headbadge during this period. The holes are consitent with Centurion's headbadge, while the chrome and wrapover stays are consistent with a Semi-Pro model.