Welcome to the forums. While the pictures are quite dark and difficult to make out details, the bicycle has traits of an entry level model, in that it has stem mounted shift levers. stamped dropouts, a claw mount rear derailleur, steel seat post and a nutted rear derailleur. It appears to have Shimano Z-series derailleurs which, if OEM, places it no older than 1984.

On the plus side, it does appear to have aluminum rims, though 27", and an an aluminum cotterless crankset, though swaged. The deralleur cables run above the BB shell and while it top tube brake cable tunnels and a set of bottle bosses, it used a clamp style cable stop on the down tube for the shift lever cables. Again, all these fit with a mid-1980s, entry level model.

Only the last four numbers of the serial number are clearly visible. Assuming it is a Yamaguchi product, the '46' might actually be a '4G', with a light or missing 'Y' prefix. However, i it's been repainted, any decals on top of the paint are suspect. It may not be a Yamaguchi. Even if it is, Yamamguchi contact manufactured for a lot of brands besides Sekai.