Originally Posted by T-Mar

Sekai was a private label brand that had their frames contract manufactured by a number of sources. One of those sources was Miki, a Japanese manufacturer. Miki manufactured bicycles for several brands, including but not limited to, Centurion, Norco, Raleigh, Sakai, Sekai and Shogun. Your serial number matches the format used by Miki and indicates it was manufactured in the 2nd fortnight of a year ending in 1 (i.e. 1971, 1981, 1991, etc.). The exact year can typically be determined from frame features.